Depends on who's statistics you read.
IMHO I think once upon a time you could say men, hands down but anymore, eh, not so much. Probably about equal with men still having the slight edge. But no matter what if someone, guy or girl, is cheating it means the relationship is inherently flawed. You can't "take" someone away from a relationship if they're happy and nobody goes sniffing around for strange when they're content at home.