That's..pretty big.
I mean, you have urges and stuff that are really, really hard to fight off and sometimes you just can't help give it and it feels good even if it's wrong.
Try to accept that what's done is done and move on. Maybe try to focus on another guy so you don't make the same mistake twice with this man. Also, he may be feeling the same as you do - ashamed but glad that it happened. You can try talking it out and making arrangements so it won't happen again. I mean, if you were married to him, would you want him having sex with someone else? I'm hoping that you are vowing to not do that again, but if you are, try to put herself in his wife's shoes and think about how it would make you feel - how hurt you'd be. Just let this be a one time mistake - learn from it and move on. That's about all you can do I guess..