Where in the Quran does it allow sex slavery

I've seen a lot of opinions of Quranists denying the authority of hadiths, now also denying that Islam permits having sex with slaves. Their arguments make sense somehow, do you have any opinions?

Like the verse that talks about guarding your private parts, it talks about both genders that's first, and it doesn't mention sex or intimacy. Their argument also says that this includes doctors or care takers of elderly people, sick etc. That sometimes need to see your private parts to help you. Does the Quran clearly and accurately permit concubinage?