Me (22M) and GF (20F) been going out about 6 months. We went to New Years house party. I didn’t really know anyone there - was mostly old school friends of hers. Anyway, we were sitting together in the sitting room talkimg and she seemed a bit bored, looking beyond me like she was searching for someone. Anyway, she got up saying she was going to get us drinks leaving me by myself. After a while I looked up and she was sitting on the stairs with some guy I didn’t know - they were laughing, chatting and she was leaning into him and generally looking into his eyes. I didn’t think much of it cos she’s naturally friendly and v cute, so guys tend to want to monopolise her. Anyway, next time I looked they were full-on making-out, really going for it with tongues and everything. Also, her was stroking her thigh and her fingers were hovering over his lap. After about a minute he stood up to go up stairs and, like an obedient pet, she jumped up to go with him. They even stopped once on the stairs to literally grind against each other and to kiss more. After about 5 minutes she came back down again all flushed and red in the face. I asked her what the hell was all that about and she told me he was a guy she had been obsessed with all the way through school and she really thought the best way to get him out of her system was to “fuck his brains out”, and she asked me if that was OK. I could have said no but was so shocked I just said “do whatever you want to do”. Then she went back upstairs and they fucked for the whole of the rest of the party - about 3 hours (I just went home) Now it’s really wierd, I thought we were exclusive and part of me is really, really pissed-off with her. However, another big part of me gets sooo completely turned on (more thn anything I’ve seen in my life) when I think about it and I imagine them fucking in the bedroom upstairs while I was down there by myself. I even find myself thinking about the black panties she was wearing that night (that she had put on in front of me with me stupidly thinking I’d be taking it off her later) and imagining him tearing it off her instead. Anyway, I didn’t want to get into a fight with him cos she had asked my permission and I said yes. She’s fairly conventional/well educated and not usually prone to doing crazy things. Guys really seem to go mad for her though (to the point it can be a drag to be out on the town with her), so I guess she gets alot of temptation. At the same time it’s doing my head in. What should I do?