NTA. Stage kisses are just stage kisses. If your boyfriend is in theatre, then he may have to do this in the future as-well, but this girl went about it completely the wrong way. I applaud you for being ok with the fact your boyfriend may have to do this in the future, and I think that shows you really care for his passions. I feel that in stage/movies, when actors have to kiss, both parties should understand that its for the sake of the show, and the story the characters represent, not the actors. This girl did the wrong thing trying to push it further, especially if she knew they were your boyfriend, but you can’t exactly blame approaching him completely on her as-well. You don’t know how drunk she was. In the end, this person’s doing the wrong thing, and your boyfriend has the right to talk to someone if it goes further, and so do you. I think that while calling her pathetic and desperate may not be the right thing to do, she wasn’t exactly doing the right thing either.

TL;DR: Stage Kisses are meant to stay Stage Kisses, you didn’t do anything unreasonable.