Even if you meet someone to whom you think you have no previous connection, a 10-minute conversation almost always reveals that she went to high school with your college roommate, used to be on a volleyball team with that girl from your book club, and had a six-month stand with your favorite barista.
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.
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.
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.
They believe this is something everybody knows, that they're just following the rules.