<human>: Can you provide a brief overview of the text?

The other night we were hanging out with some friends and we all got pretty drunk. One of our friends who is a girl is openly bi and has expressed that she's attracted to my girlfriend, but we were all cool with it because nothing ever happened. But the other night we were all kind of passed out on a couch together with the lights off and they were sitting next to each other.

After a little while, my girlfriend asks me to go into another room and she tells me that our friend kissed her and she kissed back. She felt guilty, so she told me. I didn't act upset or anything, and I asked her if it's what she wanted to do. She wouldn't ever give me a clear answer as to whether she liked it or if she's interested in the other girl, but since she seemed so uncertain, it's only logical to assume that she is interested but doesn't want to hurt my feelings. 

Honestly, I am pretty upset that she kissed another person, but I haven't really brought it up yet. I've always said that even kissing another person would be an instant deal breaker for me, but at the same time I have been dating her for over 3 years and do love her. I'm not sure if our relationship is worth throwing away because she had a drunken attraction to another girl. 

So what should I do? I know nothing serious happened, but I can't help but wonder what would've happened if I hadn't been there. It definitely raises serious trust issues, and I've always been adamant about my stance on cheating, even if it seems harmless. Any advice? Am I overreacting? What do I say to my girlfriend?
<bot>: my girlfriend got drunk and kissed another girl while I was passed out next to her, felt bad about it, and instantly told me. I feel hurt but don't know how to handle the situation.