I think you need to know more about the 'affair' before you make any decisions whatsoever. for me it's not the physical act of sex that would matter, indeed I have a pretty tolerant view of extra marital sex alone as did my ex, but should either of us ever had a deep seated emotional attachment to anyone else, that would have broken my heart. You need to live by your own morals, how do YOU feel about it, once you learn the whole story. Be aware that in these circumstances people rarely tell the whole truth on any side, so take your time, weigh things up and then decide how you will move on either together or seperately, but whatever you do, don't be tempted by a knee jerk desire to hurt and do something self destructive, just think and give yourself time and talk, a LOT.