What he did was wrong, obviously. I can't tell you what to do, but here are some things to think about.
One, we all screw up, and who among us hasn't done something idiotic under the influence?
Two, he chose to tell you about the incident. He didn't have to; would you have found out otherwise? This sounds like the behavior of a genuinely regretful person.
Three, you love the guy. Part of love is forgiveness. I fully understand that you might not be able to forgive him. That is your right. But what would you want in his shoes?
Four, you are in a long-distance relationship, and LDRs are hard for numerous reasons, including the fact that you both have a significant other that you can't see or touch most of the time. So one of you goes out drinking, overdoes it a little, starts feeling lonely and horny, and ends up making out with someone else. The LDR doesn't make such behavior okay, but it certainly makes it understandable.
Five, is this the first time he's done something like this, or the third or fourth or fifth time? Is it a pattern or an anomaly? Patterns rarely change, but a single mistake can sometimes be enough to teach the screwer-upper a lesson, severe consequences or not.
These are the things that I think are relevant here, but only you can decide if you will be able to let this go and trust your guy again. I'm a big fan of forgiveness, but I also know that forgiveness is a lot easier in theory than in reality. In your shoes, I'm not sure how I would feel and if I would be able to let it go or not. Only you can make that call.
Thanks for the question.
The Wise-Ass brought up good points. The thing I would look at if this is a one-time thing. A one time mess up, and that he told you when likely you would have not found out suggest to me also he is being sincere.
Should mention, at college parties, a group of drunk men often edge each other on to do things. He may not have kissed a woman otherwise, drunk or sober, without 'frat boys' cheering him on. Nonetheless, alcohol is not an excuse.
Still, before fully forgiving him, I'd seek assurances that he not set himself up in similar situations, or a pattern in fact may develop. Apologizing implies a willingness to change the behavior that led to the wrong.
For you Wise Ass and Mr. X -> Brilliant. :)
Well said, Mr. X, and good points all. Thanks for the comment.
Thank you for answering!!! This was the first time he's done something like this, and I was so shocked! I had no idea how to even start trying to fix things. Thank you soo much for your answer, really. This was extremely helpful!
Good. Thanks for the feedback.