That depends on your relationship. Sometimes young people make mistakes, but now it's your time to decide whether this is acceptable in your relationship or not, and be consistent. You are entitled to be mad about it if you're in an exclusive relationship. You can let it be water under the bridge; but make sure the rules are clear. (and by rules I don't mean that person is obligated to follow them, only that you're establishing what actions will have consequences with you. It is ultimately that person's choice in the end.) If she says it meant nothing; maybe she was just satisfying a curiosity and blaming it on the booze. The issue will only become as big as you make it; so if you're not angry about it as long as it's not a repeat, then say so, and be clear, and remember what you said.
Well #1 what can you do ? nothing really you can be thankful that you have an honest girlfriend who told you and at least it was another female also if it was one kiss when she was drunk .then it probably didnt mean much so its nothing to worry about
Depends on you. Just asking this question means that there's a lack of trust. She's sounds like a fun loving person. And you sound more traditional. Nothing wrong with any one of those things. But I'm not sure that you're going to be able to handle this in the long run. The times are changing and this is just what party girls do. I'm on your side a bit, though. I'm a traditional but open minded kinda guy. In the middle. That's why I like my women a little older and partied out! That said, I think you should be honest to her about what happened that night and go from there. Be adult about it. Explain calmly to her that you don't approve. If she's wants to move forward as a couple, she'll do it. And so will you. Set the boundaries and be happy together. Or do both of yourselves a favor and break up. Good luck no matter what happens!
Ignore it at least she has a good excuse that she was drunk and just be glad it wasn't spin the bottle or truth or dare or any stupid games like that where she would have a choice to say no and not do it.