AITA for not kicking out my roommate just because my girlfriend thinks he might be trans?

I (22M) have a roommate (let’s call him Alex M23) who moved in about six months ago. I honestly never considered Alex may be trans, not that I would care if he was, but that's not the issue. He is a short guy and probably under 165cm/5'5, has a lot facial hair, muscles, and looks a lot like a short Henry Cavill imo. No one I know has ever brought up this idea before, I've had my friends and family at our apartment before. This is really the part that gets to me because my mom is extremely against any gay people and if she sensed anything was up she would've caused problems right away.

Alex and I get along, we're polite but not really friends, he’s quiet but super polite, always pays rent on time, helps with chores, and even shares his cooking with me. I appreciate having him around, especially because my last three roommates were each their own horror story.

The issue came up when my girlfriend (let’s call her Sarah F28) came over one day. Alex was shirtless, to clarify I forgot to tell Alex that she was coming over, and she noticed the scars on his chest. After that she was quiet and short with me her entire stay there. When she got home, she blew up my phone, asking why I had a “female” living with me. I was confused and asked what she was on about. She says that she knows that his scars are from "top surgery" and that he is short, so he has to be trans, and a "born female".

I tried to explain that even if Alex is trans or a "born female" that there is no way I'd be attracted to him because to any person who looked at him, you would see a freaking guy. Plus he’s respectful and doesn't cause drama like my last roommates, which she knows about.

Just to be clear. I honestly still have no idea if Alex is even trans, I googled it, and those scars could be from some other surgery. Like heart surgery or gynecomastia. And I really don't have an argument for him being short, but there is a lot of short men. At first Sarah wanted me to just ask Alex if he was trans, which why the fuck would I do that, or give her his last name so she can run a background check?! I said no to both. Then she said this was a violation of trust and that if I didn't either find out it Alex is trans (and kick him out) or just kick him out that she would have to "reevaluate things". Basically threatening to break up. I said I don't do ultimatums and that we're done.

Since then, she's been messaging me every single day for over two weeks, even after I blocked her on everything because she wouldn't leave me alone, pissed that I wouldn't do this small thing for her. She ranges from, "are you fucking him?", "let's just talk", "why cant you at least give me closure and ask him?" to the most recent her telling our mutual friend about the situation. Our friend wants nothing apart of this shit show.

I didn't feel bad at first but after talking about it online, I've had some people say I should've just asked my roommate if he was indeed trans just to keep the peace, or that I shouldn't have essentially picked my roommate who've I've only had for about six months over my girlfriend of five years. I wonder if I am being unreasonable. I legitimately do not see how any straight dude could find Alex attractive, personally, but maybe I should've done something just to keep the peace.

Tldr: My now ex girlfriend thinks that my roommate is trans, told me to find out for sure or kick him out. I refused and broke up with her. AITA?

Edit, to answer some questions:

Did you break up with her? Yes. During the text conversation we broke up. I always told her I had one rule, that I don't do ultimatums. If she were to say "choose x or me" that I would leave. I put up with a lot of shit verbal and physical, but I don't put up with that kind of bs.

Ages? I was 17 and she was 22/23 when we got together. It's been a long time so I'd have to look back to make sure. But yeah, I was for sure 17. We got together the day I turned 17, our anniversary is my birthday. We couldn't get together before then because of the age of consent in my state, which I get now is really fucked up. I don't know if it helps, but we have known each other our entire lives. My mom is her mom's best friend. When my mom worked, I would go over to Sarah's mom's house so I wasn't alone. We started talking and flirting when I was about 15 or 16 but didn't cross any physical lines until I turned 17 because I didn't want her to get arrested. I get that sounds bad. I really do. But at the time I didn't see it as bad. Just in case it is asked, our mom's encouraged it.

Why would you want to be with someone like that? I don't, I really don't. I didn't realize it was transphobia until some people here talked to me about it. I thought it was just her being jealous. But I get how fucked up it is now. Please understand I live in the Bible belt, I didn't even know trans people existed until I was 16. My person thoughts is that I don't see a problem with people being trans and transitioning, I think at the end of the day it isn't my business.

Is Alex trans? I have no clue. He could be, but he could have also had breast cancer, gyno, heart, lung, or any kind of other surgery. I used a photo from Google/Reddit because this whole time I personally thought he had gyno or something. But it's not my business.

Is Alex safe? I'll talk to him when I get home and then talk to my landlord. I will change my gate code and also have her removed from the allowed guests list and also ask my landlord to not let her in personally. She hasn't been too violent of a person in the past but I also didn't know she was this insane in the past either.

Was there abuse? I feel like this has been kinda implied in some questions. I don't know. Has she insulted me? Yes. Has she been physical? Yes. But nothing crazy. Slapping, pushing, shoving, but never anything like punching or drawing blood.

Why use CM if you're American? I was born and raised American. However, I got a couple of friends who use metric from college, and after sharing a group chat with them for so long, the habit has stuck. If anyone cares, we're in automotive engineering.

The photo? The photo is not actually Alex. I searched Google for gyno surgery photos and then found a reddit post talking about it. I used it as a reference for what I mean. Scarring under the chest and around the nipple area. I definitely wouldn't actually post a photo of Alex here, censored or not. I'm sorry for confusion. Here is the source for full transparency: https://www.reddit.com/r/gynecomastia/comments/17e4ed7/examples_of_gyno_surgery_scars_from_plastic/

Why didn't you ask Alex about his scars? I have a few reasons, I personally wouldn't like it if someone asked me. Second, my mom has scars all around her body for different reasons and gets livid if you ask her about them. Third, probably the one that confuses people the most, I didn't really care enough to ask. I was curious but not I just thought "huh" and then went on with my business.

Small Update:

I talked to Alex. I got advice saying to be upfront and tell him what's up completely, hide the trans part, and that I just shouldn't tell him.

I don't know if this was the right thing but I just told him, because once I was face to face with him I couldn't really help but do it.

To clarify, I did not ask him about his scars or mention that specifically. I said my ex girlfriend was under the impression he was a trans person, made sure to say I didn't care if he was or wasn't, and that I broke things off, changed the gate codes, put her on the do not let in list, all that drama. Before even saying anything, he asked if I was okay, like I said he is a chill dude. He also not-so-subtely asked the same questions that a lot of comments asked, essentially if I was in an abusive situation. I told him I don't know but whatever kind of situation it was, it's over. The thing that really kinda fucked with me is that he called me his best friend, I regret not saying we were close in other comments. I realize now we have different definitions of close because he is introverted and I'm not. We talked about irrelevant stuff for a while and then the question came up, "would you care if I was trans?" To summarize things, yes, Alex is "trans masc". He had top surgery when he was 19 and has been on hormones since he was 18, he even has a tattoo with the date he started testosterone. While the idea that he could've been a dude with gyno, cancer, or something else is completely reasonable, it just happens that Alex is trans. And I don't care about that, Alex is Alex.

I did show him the post and got permission to update things. I would not have otherwise. He is also roaming this post somewhere, but probably won't comment.

Notes:

Alex is going to help me out with finding some low cost or pay scale therapy because he personally hasn't heard good things about the college's therapy services. Like everyone else has said, yes. It was abuse. I see that. I will also hold higher standards for myself in the future. Alex sent me the information for the therapist he sees and I'll contact them in the morning.

The landlord knows there is a domestic incident and I trust him when it comes to making sure my ex doesn't show up. The do not allow list was made in mind for this reason.

I am not ready to talk to my mom about this. But I hope with some therapy and time I will be. She knows something is going on, but she believes this is a break and not a break up.

Sorry if this sounds like rambling, it is. This has been a rough couple of weeks, my brain is fried and I'm tired. Keep in mind, I'm still a full time student during this. I also have to keep my grades up for my grants, scholarships, government aid, etc.

I do read all comments, even the not so good ones. I will try to respond more before I sleep tonight, but just know even if I don't reply, I have read it. I appreciate all the advice, kicks in the rear, and the sympathy.

A side note, I have seen a lot of trans people comment on this post and I have had a few reach out to me in private. I am thankful for your comments as well, it has brought to my attention how tough things are out there because I honestly felt what I did was the bare minimum and not worthy of praise because it should just be expected. But I see that it is being praised for how low of a bar there is when it comes to human decency towards you, and I'm sorry for that and hope things get better.

Tldr: Girlfriend of five years wanted me to kick out my chill roommate of six months because he is trans, which apparently means I'll sleep with him? Broke up with her, kept the roommate.

OFFICIAL UPDATE:

I talked to the therapist Alex recommended, normally I would be on the wait list until January, but due to the situation the therapist referred me to one of his associates and I'll be seen as early as next week. I also was recommended to attend to attend a domestic violence support group that gathers once a month, I was originally not going to go because the idea was uncomfortable as fuck, but Alex said he'll go with me so at least I'll know someone there and we can leave if it's too weird for me.

My mom is aware of the breakup, she is not too happy. I did not mention the trans part, I said that Sarah was being controlling and I didn't want to put up with it anymore. Got the usual, "that's a normal part of any relationship" comments but I stood my ground. My mom seems to be under the same delusion as Sarah that this is a small argument or something and we will get back together. But that is absolutely no happening. I don't really talk to Sarah's mom, so I don't know her thoughts on the matter.

The landlord is aware of the situation and will not let Sarah in, should she try to show up. If you don't have the gate code, you have to go to the main office and the employee (landlord's son) will buzz the person in if they are on your accepted people list or call the tenant and ask if they aren't on the list. If the individual is on the do not let in list, especially if it involves a criminal matter, they will be asked to leave. If they don't leave, then it becomes trespassing. I know this sounds like a lot, but the security is why a lot of people live here.

Notes after reading some comments:

I am sorry for trivializing my abuse, it still feels weird to say abuse, but I do know that it is abuse. Slapping, pushing, and shoving is physical abuse and if another person came up and told me their partner was doing that, I would call it abuse. It's not that I don't think women can be abusive, but as I said in one of the comments, I don't view it as abuse when it's towards me. This is probably due to being abused by my mom. Which I am going to get help for. (For reference when I mention my mom's abuse, it's why I'm low contact with her. The reason why I was extremely underweight as a kid is because she just straight up didn't feed me a lot of the time. CPS got called a lot, but never did anything. They also didn't take any claims by my teachers that I was being abused as creditable because there were no marks or bruises, which has warped my view on abuse.)

Going forward, if I date again, then I won't put up with any physical or verbal abuse. A couple of comments helped by saying that if I was confused on if it was wrong, to think about if it was another person going through it. Which has been helpful in what I feel is right and wrong treatment towards me.

After a lot of talking, it turns out Alex and I have been friends this whole time but because I've never had an introvert friend before, I didn't exactly realize. Alex says he didn't want to annoy me by trying to chat with me all the time, which ironically is what I was feeling. Now we're going to hang out more and he is trying to get me to join the D&D game he is in.

Thank you for reading my giant block of text, I'm on the app and I don't exactly know how to format. I'm grateful for all the comments, regardless if they are good or bad. I do read all of them.