If someone is willing to sell their body, even if they're amply compensated, and someone is willing to buy, does that mean it's morally okay?

Yes.

What if those sales reflect a mindset (young women are primarily sex objects) that leads us to widespread sexism, domestic abuse, rapes, exploitation of children, rotten social policies, and other crummy things?

I wouldn't accept your premise that it reflects a mindset that young women are primarily sex objects -- though they are (like all human beings) sexual. This confusion between attraction and objectification permeates the "anti-porn" crusades, and it's unfortunate. I hope you'll accept that seeing a beautiful woman and feeling attraction for her is not the same as seeing a beautiful table. What makes it different? The opposite of objectification -- and the reason you see those cheesy bits in Playboy about the model's dislikes: the desire to feel attraction to another human being. This is considered healthy in women, but unhealthy in men -- indeed, you blame it for "domestic abuse," "rapes," "exploitation of children" (?), "rotten social policies" (??), and "other crummy things." I'd call that a non-sequitur.

There is a subculture in our society that values young women, first and foremost, for how much they sexually appeal to men.

Indeed. And there's a subculture in our society that values men, first and foremost, for how hard they can hit a guy holding a ball. In our jobs we are not all valued, first and foremost, for our beautiful true selves. A job doesn't have to involve the consumers weepily coming to understand who you really are as a person in order to buy a hamburger -- it's ok if they value you purely because you can sell them a Big Mac.

We assume that sexual appeal is somehow different -- that it's wrong for sexuality to be valued financially in the same way as strength, intelligence, or willingness to man a fry station are. But why? To me, it seems like the only reason would be a belief that sex is somehow pure or special, or that women's bodies are special and deserve special protections from the hard, cruel world. I'd disagree -- a woman's body is as pure and special as she cares to make it, and she can make her own choices about such things.

This path - of cosmetic surgery, revealing clothes, coarse behavior - is hard on many of the women that follow it, because it is based on transient and superficial qualities. It tells women not to value who they are or what they want, but to measure achievement by how much they can please a man.

So it's not a path you'd choose, then? I can certainly accept that -- I would never become a fireman. The days of breathing smoke, running up stairs, and saving/losing lives are hard on many of the men who do it. So is acting -- which demands the same cosmetic surgery whether in porn or otherwise. So is slinging burgers. We'd all love to be free from such demands -- but no one is.

But porn does not tell women to value who they are by how much they can please a man. It, like any job, tells them to value who they are by how successful they are at separating customers from cash. We shouldn't confuse a woman whose goals are to please men (though, to the extent that is someone's goal...so what? They can't enjoy their job just because they...happen to enjoy their job?) with a woman whose goal is making money, and whose path to that is making men happy. In this respect, the latter woman doesn't vary much from a guy working at Wendy's -- his goal is also to please a man in exchange for cash, albeit with a tasty Frosty rather than a steamy sex scene. (The woman in the former example is like a millionaire working at Wendy's -- some people just love the job, and that's ok.)

So I struggled for a while to figure out the difference between that guy slinging Frosty's and the porn actress who is (unbeknownst to her) in psychic pain from her job. What I finally determined is that it's not about anything having to do with the job, nor is it any sort of nebulous and likely non-existent social effect (no matter how much male desire is demonized). I think it's about sexual mores. Porn is disgusting to some people. Some people like it, some people hate it -- and that hate comes from all sorts of different places (beliefs in the sacredness of women, religious beliefs, a preference for more subtle sexuality, etc.). Dressing up that disgust in moral outrage gives it legs, but in the end it'a just a sexual preference, and like all such preferences should govern only the holder of it.