You label a different behaviour than you boyish etc, but really its just a question of choice.
That's what it is in my experience. Same with being unfaithful to your partner, which I've done almost as a sport, I guess it takes hurting people who care about you enough before you maybe realize it. You being in a general sense.
But make your own experiences, I'm just telling my opinion. And maybe it's also because women who are hitting on you while they have boyfriends are generally a big turn-off for me.
Xant, that isn't untrue, but I think you still bear some responsibility if you know she is already engaged with someone else, in the end you decide if it happens or not, and I find that morally questionable. I believe a man should have integrity and discipline, to me that is a part of what makes a man a man.