Most Helpful Guy
The simple answer would be that he's not trustworthy and you should move on.
The sympathetic answer would be to not have sex till they're actually separated. I don't think you have to wait for the divorce to be legal, but actual separation. If you can't meet his (legal) wife and have her know you're his new girlfriend without this being a shocking revelation, they're not separated.
Your family will freak out, but whatever.