Exactly. My personal experience suggests that when an otherwise decent moral woman cheats on her husband, the husband is seriously dropping the ball. It doesn't just happen out of thin air. In most of the cases I am aware of where nice conservative women cheated on their husbands, it was following years of fundamental relationship problems in the marriage that for whatever reason could not be resolved.
I'm not so sure about guys though. Many men given an opportunity they can't pass up will drop their drawers even if they are happy with their marriage, IF they think they won't get caught. However, I should qualify this. If a man really is happy in his relationship, many WILL pass up a fine opportunity because the odds of getting caught aren't worth risking his current relationship. If a man is unhappy with his relationship, then it is almost a given that he will take the opportunity given the option.
I would sum the generalizations up as follows: men will cheat by default unless given a good reason not to. Women won't cheat by default unless given a good reason that they should.