Dating her best friend
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.
Of course, if your sweetie gives you a legitimate reason to believe he's untrustworthy, get out of there stat, but if there's really nothing wrong, don't create problems where none exist. Likewise, don't grill your boyfriend on what went wrong or insist that he account for his behavior throughout the entire time they dated.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.