Sexual cues are interpreted by a man (at least subconsciously) as an invitation to sexual intimacy, with all of it pleasures. This invitation makes us feel desired and desirable, and causes us to anticipate the pleasures of a sexual relationship. No wonder we like to look. Some sexual cues are probably naturally programmed into a man's brain. These include such things as
very smooth skin with uniform tone as is typical of glamour photography;
a flirtatious look or facial expression, such as parted lips or a longing gaze;
flirtatious body language, such as cocked hips or thrust out chest;
partially or barely hidden parts of the body that are not normally seen (e.g. cleavage, very short skirt, etc.)
Men are generally more responsive to visual sexual stimuli (cues) than are women ("Men and women differ in amygdala response to visual sexual stimuli." Nature Neuroscience 7:411-416 [2004]), and natural sexual cues have a beneficial function in a healthy marriage: to turn a man on at a time that is convenient for his wife. Consider, for instance, that the time a woman's skin looks most like the silky skin in a glamour photo is when the lights are low and you are so close to the woman that your eyes can't quite focus. In a primitive culture without artificial lighting, this would be in the late evening or early morning as you are lying next to your wife: a good time to get turned on for sex. Similarly, the flirtatious looks and body language, and the teasers with intimate body parts, are a natural way for a woman to communicate to her husband that she is in the mood for love play. These cues help the husband to get in the mood when the wife is in the mood, and thus avoid frustration for both.
In a world without pornography, where a man's wife is the only woman he would typically see in sexual situations, this process of conditioning would probably serve to make him more turned on by her particular features and habits, thus strengthening his attraction to her even as her body changes with age. In our highly sexualized western culture where a man sees many sexualized images of other women, sexual cues are created that have little or nothing to do with a man's wife, and are more likely to detract from the marriage relationship.