Lots of people have told me unequivocally that they would never date a friend's ex. They wholeheartedly believe that it's wrong, disrespectful, and if a friend did that to them, they'd never talk to that person again. 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. 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.
Girl code mandates that you never date your ex's friend. I mean, we've heard the charming phrases like "bros before hos" and "chicks over dicks" so frequently that the notion is ingrained into our minds: Don't date your ex's buddy and don't date your friend's ex. Then again, sometimes love just happens. Without rhyme, reason, logic and common sense, we often find ourselves inexplicably attracted to someone. And these feelings can be rare.
Which is, of course, the people you already know. They likely know the restaurants you like and some of your Game of Thrones fan theories — they also probably know all the gory details about you and your ex. There are pros and cons to all of these scenarios. Who cares about what other people think?