I agree men have the monopoly (but not the exclusive rights) on meaningless sex. Sooo.... if for women an affair is more a mind thing - the flatterey, attention, novelty, etc - are you just as guilty of being unfaithful if you spend husband time on day dreaming about mr other, whether it's robbie williams or your husband's younger brother?
I cannot think of an argument that logically lets the day dreamer one off the hook. OK, you are not being physically unfaithful, but you are being mentally unfaithful. You are still neglecting your relationship, ignoring your marriage vows, and, presumably, you would not want your children to find out.
So, playing devil's advocate, assuming you have been mentally unfaithful on the sly many times in your marriage, why is it such a big step being physically unfaithful too, as long as you are not found out?
It could even be argued that having a ongoing, secret affair with someone (that stops at this by mutual agreement) frees you from spending vast amounts of time yearning for them. Three hours once a month being physically unfaithful as opposed to three hours a day being mentally unfaithful.
On the other hand, if you agree that any unfaithfulness is wrong, then should you make a real effort to stop yourself thinking about other men?