Besides morals, it also has to do with self-control.
Yes, it's human nature to find various people attractive. You're not going to walk around with your eyes closed after marriage. And as someone mentioned in another thread.........the opposite sex will not become less appealing to you after marriage. But even if you do think that another man/woman is attractive.........have some respect for your spouse......and don't act on it. Some people might say that "flirting" is not "acting".........but I think it is. You're putting your thoughts and feelings into actions.....the action being the unnecessary banter or the continuous gawking/oggling. Sure......it's not as severe an action as cheating or a full-fledged affair........but you open the door a crack.........and it has the potential to open all the way.
Sometimes I think that we try to justify our actions by saying that we'd be "okay" "totally cool/fine" if someone else places us in the same situation. Or we justify it by saying, "it's natural.....no harm in it....human nature....everyone does it"........but then the fact that we don't have to courage to do the action OPENLY before those that we're hiding from.......is an indication that you're not 100% comfortable with your actions. Also, if you have to placate your conscience so many times regarding an action........then maybe it calls for some reflection about whether or not it is truly "okay."
We get tempted by many things.....some people have better discipline than others. But the stories that you hear these days (and technology....internet....texting has made it easier to cheat)......can make one question the point or sanctity of marriage.