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.
We know our backstories will be tangled and intertwined.
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.
RELATED:22 Reasons to Stop Worrying About His Ex-Girlfriend17 Things I Wish I'd Known About Getting Over an Ex When I Was Younger11 Reasons Why He Broke Up With You Follow Lindsay on Twitter.
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Whether you're gay, straight, bi, or not into labels, dating a friend's ex can absolutely be done without sacrificing your friendship — you just have to follow a few simple guidelines. (I'm going to use female pronouns for your friend, and male pronouns for your sweetie, for the sake of simplicity; however, every rule here applies no matter the genders of the participants.) Keep your friend's secrets.
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.
But every once in a while, the universe speaks to a person and lets him know that, although it seems wrong at first, there might be a bigger reason your friend dated this person in the first place – maybe it was to connect the two of you, instead.
Such a situation, of course, can be tricky, and must be handled with care.
This rule is almost never stated or enforced among queer communities.