I'd agree with your girlfriend. So long as there is no deeper meaning behind he kissing, then its not an issue.
What may be an issue is you not having had the chance to talk about how *you* feel about it, and ask that she not partake in these activities, before she was in a situation where it may have happened.
You should probably seriously tell her if you are that uncomfortable with her kissing other people. Saying you don't intend to makeout with girls isn't the same as asking her not to make out with other people.
It's only doomed if you can't find a compromise, or cant trust her.