Men are Hardwired to Look.
What happenes after that is up to him.
I did a study on this some time ago, it was not a very big study....but enough evidence to know that it is almost impossible for most men, not to look at attractive women.
If all is not well in his marriage, or relationship....if a woman was to think he too is attractive (and maybe single) it can and in some cases, cause him to 'think' about her.
Women don't have the luxury of even looking at other men, their instincts are more geared up to the children they have, and having a happy home life.
That is not to say women do not cheat......they can and do.