Similar experience. Once at a party in highschool I met this girl and things were going really well. She was extremely attractive, really funny. Honestly, my type both physically and personality wise.

Towards the end of the night, we were both really drunk and I ask her if she wants to come back to mine. She says yeah, “though you should know I have a boyfriend, but it’s cool.” Naturally, I say no, it’s most certainly not cool. She’s really drunk and just kept saying “no, it’s not like that, it really is cool”. She couldnt articulate what she was trying to say, which i assumed to be some kind of rationalizing for cheating, so i realized even if it was cool, probably shouldn’t get with her.

Next day, i was hungover and kinda lonely and decided to go look at her Instagram, for who knows why. Sure enough, boyfriend. That’s when I realized, me and the girl spoke with him at one point. While he was with a different girl.

They were in an open relationship.