How This Affects You
Men have less of certain hormones and more of others than women do, and that fuels us to respond in different ways. For most women, sex is primarily an emotional thing; for most men, it's primarily physical.
A man often fails to see how sex and other aspects of the relationship are intertwined. I have always counseled women that there are times you make love, and there are times when it's purely recreational: Wham, bam, thank you ma'am—just a complete physical release, and cover me up when you're done. Men and women both need to recognize that there's a range of feeling when it comes to sex. If you don't acknowledge that range, the two of you are going to have a problem.
There is definitely a double standard with men when it comes to fidelity. If they are having an extramarital affair or an extra-relational affair, they'll probably look at it as if they are going and taking something from a woman, not giving something of themselves away. There's no emotional investment there.
Men are visually stimulated, which means if they are in a target-rich environment, they may well become aroused. This is not just a maturity issue; their brains are actually wired that way, which is very different from your own wiring. But this is not some involuntary reflex action over which he has no control. It is a choice. Men can be amazingly shortsighted on these issues, often failing to project ahead to the consequences of their actions on their wife or children.
What You Can Do About It
Discussing the matter of fidelity, before a crisis occurs, is crucial. Couples need to talk about what fidelity means to each of them so there's no confusion. Understanding that men fear rejection and therefore thrive on acceptance and are easily visually stimulated, you can program out a considerable risk of unfaithfulness by investing energy in what may seem superficial: your appearance and the level of sexual activity in your relationship. Please don't write to me objecting! I'm not saying this is fair, only that it is effective. Let the training begin.