<human>: Can you condense the text into a few key points?

Hi all, using a throw away because people know my other account. And please dont say its awesome that they kissed. Be mature. 

Let me start by I dont know if I am over reacting, if i am right or wrong or just plan confused as this has never happened to me.

Few days ago my girlfriend and 3 of her friends (2 guys 1 girl / her bestfriend) went to a concert. Basically we drank at my house since concert alcohol is expensive as hell and I had work at 5 am the next morning so I was the DD. And by drank at my house I mean these 3 got smashed.

To make this super short before the main person coming out (intro music or whatever) her and the other girl were dancing. I was sitting down finishing my smoke and then I saw it. They kissed. Not like that passion make out but they kissed a lot. No tongue that I saw. 

Now is this cheating? Does the fact that it involved alcohol make it worse? How can I trust her when she goes out and doesnt kiss guys or girls again while being under the influence? Im confused i never had this happen to me or at least not in person. Do I bring it up or let it go? I feel like she violated my trust.
<bot>: Girlfriend got drunk was making out with her friend and i dont know what to do. And no girls kissing other girls is not awesome, its still wrong and not an excuse.