The Seventh Commandment--or if you are Catholic or a member of certain Protestant bodies, the Sixth--as spoken by God to Moses on Mount Sinai in Exodus 20. Well-to-do jackass Ted Turner once joked during a speech that this should be thrown out of the Ten Commandments, and unsurprisingly found himself divorced not too long afterward. This is one of those we sometimes wish weren't in place, particularly when Jesus went and internalized it by declaring that if a man looked on a woman with lust in his heart, he had committed adultery with that woman. In a word, yikes.
The foundation of human society is the relationship between a man and a woman. There are obviously other kinds of relationships, worthy ones, but this is the one that generates new life. More than any other, it needs great care because damage done to such a relationship spills over into all other parts of society. Rabbi Shmuley Boteach wrote in Kosher Sex, "If people who once professed love to each other and joined together in matrimony cannot live together in harmony, what hope is there for more far-reaching and abstract goals like world peace?" Adultery is choosing what's convenient and frivolous over something the rewards of which require true and lasting commitment. It's bad for your soul; it's bad for your marriage; it's bad for your children, bad for your neighborhood, bad for the world.
It also makes you feel guilty as hell, which is bad. If it doesn't make you feel guilty as hell...dude, that's worse.