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.
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.
Save it for your diary or for anyone who didn't date him. This can be extremely tempting if they ended on bad terms and you know you'll find a sympathetic ear.
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 4. Don't ask your man if you're prettier/smarter/better at Scrabble than his last girlfriend.
Don't do this ever, but especially not if his last girlfriend is the person you're going rock climbing with Sunday.
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.