I cheated on my ex.
I had three ONS.
I was very young. My ex wasn't a great boyfriend (I did stay with him for nearly ten years though, and he was abusive - physically, emotionally and financially).
I was only 19 when we got together. The ONS were at the start of the relationship. One was with someone I'd had the biggest crush on during high school... it was crap!!!
I would never cheat on my dh. Ever. Our relationship is built on mutual trust, respect and love. So, I don't necessarily think once a cheater, always a cheater... but I think it depends on the relationship. The one I had with my ex was all kinds of wrong, right from the start. I can see that now, because I've had five years of a happy relationship to understand what was wrong in the previous one.
However, I do think some men will always cheat. I know this sounds contradictory, but I think there's a lot of difference between a young person experimenting and experiencing a few ONS and getting involved with another person on an emotional level. Men who cheat are like this... they get involved emotionally as well as sexually and that to me is worse.
I'm not condoning my behaviour when I was young, but I think it's a different pattern to a serial cheater. IYSWIM?