Women who accept being a mistress - what do we think’s going on here?

I’m still working through the aftermath of being cheated on. The woman he cheated with knew about me and when I confronted her she said my relationship was nothing to do with her and it was his responsibility to tell me. He said he did not lie about our relationship to her (there were no issues I was aware of) and when I asked he said she didn’t have a problem with the situation.

They’re together now as far as I’m aware. He said he didn’t tell me at the start of the affair because he wanted to see if it worked out with her (he’s a peach right?). We were together a long long time, living together etc. which she knew about as we had her children over for a sleepover and parties many years ago.

I’m wondering if you have any insights about what type of woman would do this and why? No name calling e.g. ‘coz she’s a skank’ as I already told her about herself 😂. But i’m a lot calmer now as it’s been a few months so just trying to get a broader perspective. Thanks!