I agree with the previous posts..

Cheating is bad, affairs are wrong, and if there are kids involved, then the person might consider that there are things beyond their personal wants like the children involved.

Another side note is, if this man is willing to have an affair, what does this say about his integrity (and this could go for women too)? Is these the qualities and values you really look for in a person?
If they do this to this person, what makes you think they won't do it to you? We all like to think that it'll be different, its not the same etc etc. But remember on what you build that relationship on.