So you think women are fully aware their husbands (or boyfriends) think about doing other women all the time?
In the back of their minds perhaps. But it is of course unhealthy to think about it if it bothers you, since no woman will ever find a normal straight male who doesn't.
Women do it also, just not so much and perhaps not so "tuned in" to every good looking woman they see.
It is really quite fascinating. You take a large crowd of people, say at a basketball game where you can see most of the people on the opposite side of the court. In that crowd let's say there is a really attractive woman. Just about every man on the other side saw her within seconds of her arrival...even if not one of them were looking for women to look at.
Women think we are always "looking". We are not. Not intentionally most of the time. It is just that our brains are so hard-wired to copulate and spread our seed like every male species on the planet - that we truly can't help noticing every attractive female within eyeshot. The trouble begins when the woman tries to personalize it.