Really no line for me personally I've got pretty thick skin. There's never been anything someone could say to me personally to truly offend me or get me fired up. I'm the kind of person to just fire back slick talk and get the other person more fired up and get under their skin.
However if someone were to say something about my girlfriend while we are around and she took offense I would have to nip it in the butt quick. That doesn't mean go up and argue or fight just let them know, have some respect man.