Why does TRP encourage men to have many sexual partners but encourage women to have fewer sexual partners?

Sort of.

A good looking, alpha-traits man dedicated to a single woman is a woman's fantasy. Getting that is AF+BB in one person. It's the ultimate win condition. No competition or effort for maximum reward.

Catch is, what's good for her, or good for marriage isn't the best for him. It's not a fair trade. He's better off using his status to sleep with as many as possible, leaving it up to her to win him with favours and effort. That way he keeps up the pressure on her to perform.

Sure, this works in reverse, but only in that she can show off beauty and the attention of other, high-quality men.

Why doesn't sleeping around make sense in reverse? Because sluts aren't impressive. Any dude will sleep with a chick. An attractive woman can pick a number from 0-100 and get it in a year. A dude with that same ability? He's impressive, because it's difficult.

If we think of relationships as a sex/commitment exchange, if she's already given away her sex to anyone, it's really low value sex. Why should a guy pay high price for that, when he can get it cheap?