I think everyone has a different definition of crossing a line. I don't think I have any rivals. I don't see my partner as property and I trust them. If a guy flirts very openly with them, I'm sure my partner will be nice but shut it down. If a guy becomes inappropriate (ie unwanted touching or verbal abuse) then it's a case of saying "wow, take it easy man that's a bit inappropriate". But until it reaches that point my partner doesn't need my help. And I'm not going to bash anyone over the head for trying their luck with my partner, because nothing is going to happen. And say for example a big greek god came in and my partner was overwhelmed with desire and ran away with them. Well, that sucks, but good luck to them. She made a choice, I can't control her and if she does that she's obviously not who I thought she was and therefore not worth fighting for. Very little is worth literally fighting for. We live in a society not a jungle