Magazines, the media, and even Hollywood all would have you believe that what a man really wants is what he sees. In other words, it's the woman with the sexy figure and the beautiful, flawless complexion. Women are mislead into believing that, to a man, it's all about their physical appearance.
When a woman, for example, finds out that her husband has cheated on her with the younger, and perceptibly more attractive marketing coordinator from his workplace, she automatically assumes that the reason has to be that her husband was simply more attracted to his colleague than herself. After all, she's "hot," right? But, is that what really led him astray?
Consider this. The relationship, in the eyes of this hypothetical man, was missing something. He was craving something that he felt he wasn't getting from his partner, and it quite possibly wasn't some random physical attribute that his partner was lacking. He had a need that wasn't being met, and I already know what you're assuming it is, but you're wrong.
What he craved, and what most men really want, is attention and admiration. Men want to feel important and appreciated, much like you, yourself, probably do. In the example, earlier, the man most likely was not getting any of that at home. When his colleague was more than happy to give him what he craved, it made him feel admired and boosted his ego. In short, it made him feel better about himself. She fulfilled his needs that weren't being met at home.
Now, does any of this justify the infidelity of the man in the example? Of course not. However, the lesson to be learned here is simple. Keep your man happy and his needs fulfilled, and what reason would he have to look elsewhere?