Guys check out women regardless of their lovestage.
One of the classic complaints women have about men is the never-subtle wandering eye. Whether it's checking out the cute waitress, staring at the girl at the bar or even just the too-long glances at friends, men constantly seem to be checking out the females within eyesight. Most of these men whose eyes linger lustily don't actually want to make a move, but they do enjoy seeing a nice-looking woman (remember how visual men are?). Regardless, women never seem to enjoy watching their partner watch another pretty lady. Read: To Ogle or Not To Ogle
Well, complaining may be fruitless. Science Daily reports on a study showing that men who have a sexual partner are no less interested in looking at other women than men without a sexual partner. Women with a sexual partner, on the other hand, have far less interest in other men than those who do not have a mate. This was based on how long each participant looked at pictures of the opposite sex to determine attractiveness.
All of this can probably be traced back to some sort of evolutionary attraction thing, but in today's world, women might just have to accept the fact that men may always be giving a new female face a once-over. It doesn't have to mean that they don't love their partners. Likewise, women check men out, too. They're just more subtle.
This study's helpful for another reason. Just because your friend's husband/boss/insert-other-inappropriate-example-here gives you a stare doesn't necessarily make him a pig. A once-over and a wink? Well, that's a different story. Read: The Other Woman: What A Mistress Knows