Monogamy is totally realistic, but at the same time, if a strait man (even a married one) tells you that he has not once lusted after another woman; he is not being honest with you or himself. On a basic, primitive level, men are hard-wired with a desire to "spread his seed", so to speak. It is very natural, pretty much every male animal on this planet is wired the same way; the difference is that as humans we are given a soul, a consciousness, the intellect and awareness to control theses urges...although some women would claim that we don't very well...lol!!!
But seriously though; lust is a major issue for many men, Christian or not. I love my wife very much, but if it is Summer time, and a nicely toned, half-necked college student comes jogging past me, I would be lying if I said it wouldn't capture my attention for moment.
A wise man once told me, "you can't stop the birds from flying over your head, but you CAN stop them from making a nest in your hair".
This pretty much means that as human beings, sometimes thoughts, even impure ones, will sometimes cross our mind; sometimes it cannot be helped. It is not the thought itself that is sinful, but DWELLING on the thought, and even more so, ACTING on the thought that makes it sinful.
So if you see a beautiful or attractive woman, it is okay to acknowledge her attractiveness; but do so...then move on. This can often be easier said than done.