No matter what his answer is, it's going to make things weird.Besides, comparing yourself to anybody — even if you come out ahead — is going to lead to feeling crappy, because basing your self-esteem on where you stand relative to someone else is Not Healthy.In fact, when we met, my now-partner was on a date with my best friend.
(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.
Remember that you can love them both without them necessarily having to enjoy each other.
This goes for friends and partners who haven't dated, too, now that I think of it.
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.