I'll try not to drip feed.
I live in a house share of 5 people. There is me and my partner, and three other women. All of them have boyfriends and we all get on well.
One roommate has her boyfriend over quite regularly. I heard them having an argument a couple of weeks ago, she got upset and came into my room and told me that he'd told her that he thinks he may have slept with someone else a year ago but he was too drunk to remember.
The only reason he'd told her was because it didn't feel normal down there. I told her that I would drive her straight over to the clinic to get tested so at least that would put her mind at ease.
Her boyfriend took her instead. I was texting her throughout to make sure she was ok. I was upstairs working, came downstairs and they were back and she was making him dinner and they were chatting as if nothing was wrong.
All really odd, but I feel it's because they've been together since such a young age that theres not much that would break them up and shes just accepted that he may or may not have slept with someone else.
However since then her boyfriend has completely ignored me. Will actively leave the room if I come in, will only just about say hi to me etc.
Brought it up with roommate and she said in a very matter of fact way, yes I doubt he'll ever speak to you again because he knows that you know.
I have never once mentioned it to her boyfriend ever. But I'm the one made to feel uncomfortable in my own house now.