I've heard that some men start an affair with married women because they **KNOW **that she's married and so they don't have to commit to her......thus making it easier for him to get what he wants.
And in the event that the married woman does ask for a commitment, some men will use the "marriage" as an excuse to get out of the affair if things start getting serious. "I really care about you and I feel guilty about deceiving your husband like this, bla bla bla."
Or who knows? Some men might think that a married woman might be more special than a single woman because she possessed positive qualities that made another guy want to marry her in the first place. It's like how some women might think that a guy who is already in a relationship must be special if another girl (his wife or girlfriend) has deemed him worthy enough to enter a relationship with.
Or maybe it's much simpler than we think. Maybe majority of it has to do with looks. Perhaps........it doesn't matter if she's single or married.......as long as he finds you attractive.
I highlighted a para in red. Some of my guy friends have confessed to this being a reason why they have been attracted to married women.
Sometimes it's as simple as wanting what you can't have.