I was cheated on once, my DH picked a work colleague and fed her all the right lines.
I didn't support him
We had nothing in common
The marriage was dead
We no longer had sex
All of the above was bull, and he admitted that... he just told her what he knew she wanted to hear.
The only 'problem' we had in our marriage was a lack of time. She was at work with him for 40+ hours a week, all glamorous and exciting. I was at home running around after small children and organising his life.
He and I were still shagging, going out together etc.
I'm sure that there is an adulterers handbook that all men refer too. I wouldn't go out with a married guy, it's no basis on which to start a relationship...and it never results in 'harmless fun'.