Mostly they're not. Too simplistic a question, most women desire a monogamous relationship and if given a choice between equally attractive men would pick the single one. But, big but, some men are single for a good reason while married men appear to have some aptitude. So if you have to choose between two evils you'll have more fun with the one he knows what he's doing.

Because they figure the man is already attractive to someone, has curb appeal, in other words. And they know he will likely never want a commitment from them. Then there are the women who actually want a permanent relationship with married men and suffer the illusion they will leave their wives. All just hunches on my part and second hand stories. I have never personally known anyone who is morally lax enough to knowingly date a married man.

I'm a single woman and I don't find anything appealing about married men. Any woman who gets involved with a man who cheats on his partner deserves the betrayal he's eventually going to inflict upon her.