I understand some might not be more jealous than me, but I won't fight because my girlfriend is looking at someone and finds them attractive. Some might not feel the same but to start a fight over something like that is not good. Now if they're touching my girlfriend that's a different story.
Firstly, I'm not the kind of guy that gets jealous in a relationship. If my friend were to date one of my ex-girlfriends, I'd say "Dude, how'd it go?"
But fighting over a girl is gibberish to me. She's not a prize, she is a person and if someone else were to flirt with her, all I would do is just SAY that she's with me. And if HE were to pick the fight, I wouldn't fight back.