Go talk to other girls.
I dont care whos hitting on my GF. Shes hot so guys are going to hit on her. If I worried about it I would never get anything accomplished in my life.
At a party and some dude starts talking to my GF? Maybe I'll talk to him if he seems interesting and start a convo. Maybe i'll walk away and talk to other people (including women) if im not interested in conversing with him. If hes overly annoying or lacks social grace I'd just take my GF and walk away but truthfully she would probably tell him to get lost first.
If your woman doesnt respect you then you have bigger problems than random guys hitting on her.
I'll give you a few examples of my life.
1) Last weekend I was at an art gallery with my GF. I happened to be several feet away from her talking to a potiental business associate while she was looking at a painting. Some guy walks up to her and starts chatting her up. I noticed but didn't much care. Shes a big girl and can take care of her self. Few minutes later Im done talking to to the gentlemen I was conversing with so I walk over to my woman, place my ahnd on her lower back and say "hi babe." I stick my hand out to the guy who was hitting on her and introduce myself. He was rather annoying so after a minute or so I turned to my GF and said "hey, you have got to see this" and turned back to the guy and said "it was a pleasure" and walked off with my GF with me. Simple, tactful, graceful. This will work for you almost anywhere.
2) Several months ago my GF and I were at a dinner party celebrating a promotion of one of my GFs friends. During this party some guy comes up to hit on my GF while I was talking to someone else. When I came back I introduced myself, the guy shook my hand and gave me his name as well. We got to talking and now today hes one of my best friends. He stopped hitting on her once he realized she was with someone and he turned out to be a cool guy with lots of common interest.