Im dating my ex girlfriend
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.
Set aside time for each of them and honor it — don't drag your lover along on girls' night out (not even if your lover is a lady; queer chicks are about this), and don't invite your friend to what was supposed to be a romantic dinner at home. Don't do this ever, but especially not if his last girlfriend is the person you're going rock climbing with Sunday.
No matter what his answer is, it's going to make things weird.
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.
Queers don't tend to expect our dates to come into our lives completely free of prior complication.
Their relationship is between them; it's not your cautionary tale or your soap opera. It's easier, of course, to have hard-line rules — "exes are never OK" versus "exes are totally fine" — but that's not the world we live in.
If they choose to share details with you, that's fine — you don't need to stick your fingers in your ears, unless an overt comparison is being made (see No. Your relationship and theirs are separate things, and you don't need to know anything they don't care to tell you. If someone seriously mistreated your friend (we're talking emotional or physical abuse, infidelity, lying, stealing, etc.), don't date him, no matter how awesome his butt looks in jeans.
Queer communities are often small and insular, and once you've found one, you tend to hold on to it for dear life.
It's difficult to meet people you're romantically interested in beyond an already-defined circle, and outside of your city's queer scene, most people you run into are likely to be straight.