Married with questions

I was thinking about the ten commandments the other day and got to wondering…Why is there one commandment forbidding adultery and another forbidding men to covet their neighbor’s wife? Perhaps a waste of tablet space? If there’s hell to pay for adultery, then it stands to reason that a dude is better off not even thinking about it. And if we’re not even allowed to think about it, why the prohibition against coveting your neighbor’s wife? Because what if you’re out of town on business? And what if your neighbor has a friendly unmarried sister?