I can speak from the girlfriend perspective... because I have friends that I had slept with in the past that I was still friends with on a regular basis when me and my BF got together. At first my BF said it didn't bother him but clearly it did because ever time we got into an argument it some how was brought up. The difference in my situation was that my BF was not friends with these guys just me... I did end up learning the hard way that my "friend" was still interested in me sexually and subsequently I ended the friendship, because my relationship with my BF is much more important to me.... But if you trust your GF and you trust your friend and you don't think that there is a possibility that either of them is still interested in one another then I think that you are the one that has the problem... You have to realize that if it meant nothing to your GF than it should mean nothing to you as well. At least they didn't have an emotional relationship where they were inlove with one another.. Drunk sex happens and it doesn't mean that there is any chemistry or even attraction for one another. So I hope that you are able to erase some negative images from your mind.... Just remember that she loves you
Good luck