I have found that Caucasians have a looser viewpoint on same-sex kissing. To them, it is just a drunken thing to do. No need to break up. Just speak on it, monitor and evaluate.

Is she Black?

If yes, have a serious conversation with her and ask if she has lesbian tendencies. If she does and you are cool with that, stay. If not, think about moving on. Black folk tend to be more measured with their public displays of affection. It is probably not a frivolous act.