Guys who are married or have serious girlfriends, why would you still check women out?

I work in a setting where I see a lot of married guys checking out women. For instance, my professors' husband was not in a "sexual" matter, but yes i've seen him staring at me a couple of times. For reference, my professor is a beautiful woman and I doubt he was doint it intentionally. I dont stare at him neither any men who's married, but I do wonder, why men do this even in a serious relationship.

Guys who are married or have serious girlfriends, why would you still check women out?
Post Opinion