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.
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.
This rule is almost never stated or enforced among queer communities.
If you're gay, you will almost inevitably date a friend's ex at some point.
The reverse is also true; no matter how much you love discussing your dude with your besties, his ex can probably live without hearing the details of his current sex life. It's OK to come to your partner for advice if you're arguing with your friend, or vice versa, but absolutely resist the urge to belittle or insult one of them to the other.
They wholeheartedly believe that it's wrong, disrespectful, and if a friend did that to them, they'd never talk to that person again.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.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.