PASSION is a game changer, often times, as it creates emotional attachment. A lot of folks, including myself, prefer "making love" rather than just "fucking".
That's one good reason why NOT to meet alone with a new sex partner.
With HIS being married as well, it shouldn't be all that difficult to sit and talk this one out a bit ... because once his WIFE finds out, all kinds of crap will hit the fan. And she WILL eventually find out.
Hell, maybe his wife is pretty as well ... just bored with him or he's bored with her ... why not just do some swapping if that's possible. Women often have problems sharing their own men with other women because they KNOW other women aren't to be trusted. So, knowing you might be receptive to a spousal swap for a while might just bring HER back to earth ... she'll be willing to talk THEN, I bet.