It's not a polite way of saying "I don't trust you" at all. I in fact come from this mentality in a way, although less from a being bothered about losing my partner way.

I couldn't care less if the person I'm with goes off with someone else, I would never try to stop that happening in some paranoid manner, it would just be stupid and I've explained this before. If your partner views someone else as someone they'd rather be with why would you want to be with them? I know for a fact that I'm the best person in the world in the eyes of the person I'm with. If she wants to go off with someone else I won't stop her.

HOWEVER, the issue I do get is an overwhelming sense of rage at the disrespect guys show for me by trying it on with her. It is fucking disgusting that so many people come from a mindset of believing it's ok to crack on a girl that's not single. Most of them even justify it as ok because "he's not a mate" and tell themselves he's a dick in some warped way.

This is what bothers me. Back in the day however I used to use this "everyone else" thing based on a fear of my partner finding someone better than me - not cheating on me. I never worried about being cheated on. Losing the partner to a person better than myself was however a major issue and the lack of trust for everyone else comes from the knowledge that people couldn't give a shit whether someone is taken or not.

That's what's going on here.