Im dating my ex girlfriend
One reason your ex may care if you are dating is because he still holds onto feelings for you.This may be particularly true if you were the one to end the relationship.
This goes for friends and partners who haven't dated, too, now that I think of it. Don't ask your man if you're prettier/smarter/better at Scrabble than his last girlfriend.
This has nothing to do with some kind of Eternal Dibs situation, and everything to do with the fact that, by choosing to build a relationship with someone who treated her horribly, you're telling your friend you don't think what he did to her was all that bad. There are lots of people out there who are just as good in bed and haven't traumatized anyone you care about.
Set the precedent that people who are awful to your friends are people who don't get to see you naked, and your life will be the better because of it.
They believe this is something everybody knows, that they're just following the rules.
What I've noticed, though, is that every person I've heard espouse this worldview was straight.