So I have this friend (female) and have known her for multiple years and now have a boyfriend of one and a half years who I often bring to partys so he knows my friends and interacts with them regularly. In the past I often invited this girl to his house when he had like functions there so I'd have a friend of mine there too. Because she lives a little far away from his house she would often sleep there afterwards and I would too of course. I never had a problem with that but after some time she wouldn't even really announce it to me before .Then there was one weird evening were she said that she had to stay till 5 am because that is when the first train comes because she didn't want to sleep there this time but didn't really communicate that before and wasn't really excited to stay up for that long. After some time most of the other people were gone and I was tired too but he didn't want to leave her alone so they stayed in his living room together and I went to his room. Then at 5 am I texted him were he was because he still hadn't come to bed and he said that she would stay one more hour. Mind you before that she even counted down the hours because she wanted to go home so I was really confused because at this point they spend like 3 hours down there. After that I talked to both of them about how weird I found this incident but I would never think that either of them would betray me or anything because I trust both of them but I just didn't understood that situation. So after that there have been multiple functions were they would stay together really long on functions and I always told him that it made me uncomfortable but he says that he can't really talk to anyone else there (he's a really extrovert person who likes to get to know new people) so I still don't really understand it but am I the asshole here?