Hormone modifies behavior. We're just misunderstood. Again, this is a testosterone thing. Besides, women do it as well. Women are just much better at not getting caught.
Women take one quick look and memorize it for later reference.
Since men lack this ability, we try to burn it into our memory by staring as much as we can. It's much like adjusting your bra. Being in public is just an added bonus. You'd learn to keep your big mouth shut too if every time you open it you get into trouble with your partner. Unless we're experiencing some extreme emotion like rage, hatred, disgust, or a brick on our foot, we have no idea how we feel.