While I don't technically believe in karma, as a magical force or whatever, I think it's fairly retarded to deny that what goes around, comes around. We live in the world that we choose to perpetuate, and to deny any personal responsibility in maintaining a form of social contract, e.g. agreeing to not fuck another guys long term gf, is a very short sighted, aka idiotic, form of trying to relieve oneself of the interconnection we have with what we do to others and then what happens to us and those we care about.
On the other hand, I often get pissed off when I meet lots of attractive girls who have bfs, and I'm like, is it really fair that he gets to fuck you simply because he met you first? I mean women are no ones property, and sometimes it seems like a lot of AFC soft-chair guys, who want to prematurely hang up their game cape and stick their lazy slippers on, are going around 'bagsy'ing' a women and then expecting the rest of us to respect their right of ownership. If she's willing to cheat, then perhaps he has a bit more work to do?
Really you need to assess the situation on its individual merits, and be aware that your decision will also reflect and imprint your image of how the world should be. So hopefully it will be a constructive one that doesn't come back to bite you in the arse, as others are saying.
