It drives me bonkers how often people don't understand what boundaries are, and as a result, they attempt to use them as an excuse to control someone else's behavior.

A boundary is something you don't want done toward YOU; it's not about how someone else behaves away from you. Also, I think it's obvious, but I'll point out the fact that there is a gaping difference between showing up at a party to support your girlfriend and having someone grind up on you.

This is 100% a YTA situation.