But there are lots of ways to abuse yourself, right? A steady diet of Krispy Kremes would do some serious abuse to your body (or, I should say, the body that is you – in this scheme, the body is not an owned thing that is yours). Not serious enough to warrant legal sanction? Well, gluttony is one of the seven deadlies, right up there with lust. And the medievals did have all sorts of laws regulating consumption. Are we going there? Or pick some other sex examples: I don’t believe Ross approves of legislation against sodomy, masturbation or adultery. But those are all behaviors that are traditionally understood to be “self-abusive” (indeed, what does “self-abuse” traditionally mean?), and the last of them (adultery) isn’t even arguably victimless. Why not ban them? The law can be a moral teacher, after all, even when it’s not aggressively enforced, as Ross himself says.
Why are some forms of paternalism – of which morals legislation form a sub-category – OK and others are insulting to a free people? I don’t think you can get back to an answer that prostitution is the sort of thing we should keep illegal without some argument about the power relations that predominate in the transaction in question.
I actually think it’s perfectly possible to distinguish between forms of moral paternalism without resorting to arguments about whether the behavior that's up for regulation is ultimately non-consensual. Here are four grounds one might employ: The seriousness of the form of self-abuse in question, the avoidability of the abuse (to borrow from from Aquinas's argument that the law should only prohibit vices "which it is possible for the greater part of the multitude to abstain from"), the plausibility of regulation, and the private/public distinction. So masturbation, for instance, is a private and well-nigh universal vice (among men, at least) with no plausible means of interdiction. Sodomy and adultery are likewise acts that can only be regulated through extreme invasions of privacy; moreover, the fact that homosexuality seems to be an innate orientation rather than a choice means that even if one views same-sex sodomy as immoral, it isn't plausible to expect "the greater part of the multitude" who are inclined toward homosexuality to abstain from it. (I'm stretching Aquinas's argument a bit, but not implausibly, I think.)
The possession and abuse of illegal drugs can similarly only be prohibited at a significant cost to personal privacy. I tend to think the gravity of the self-abuse involved outweighs the cost to privacy, but only in some cases: If you’re growing marijuana in your basement, the law should probably leave you alone; if you’re running a meth lab, it probably shouldn’t.
In the cases of vices that involve excess, meanwhile – gluttony and alcoholism, for instance – there’s also the problem that any plausible regulation designed to restrict the abuse would end up casting too wide a net, and infringing too far on ordinary pleasures. If bans on alcohol only affected people with debilitating alcoholism, or bans on fast food only applied the grossly overweight, I might be inclined to support them. But there’s no plausible way to make these kind of distinctions: If you ban fast food or spirits, you have to ban it for everyone, or else set up some sort of police state around liquor stores and McDonalds.
In the case of prostitution, though, none of these objections to a ban prevail. Selling or buying sex is a grave form of self-abuse, rather than a minor one. It’s a public vice, rather than a private one. It’s hard to stamp out, but you don’t have to invade people’s homes to significantly restrict it. And it’s a practice that "the greater part of the multitude" are perfectly capable of abstaining from, since there's no innate inclination toward buying or selling sex. Whereas sodomy laws, by forbidding gays and lesbians from pursuing sexual and romantic intimacy, attempt to enforce a heroic adherence to natural law, anti-prostitution laws are far more minimalist in their aims, and far less invasive. It's cruel and unreasonable for the law to demand that Andrew forswear sexual relations with the man he loves; it's perfectly reasonable, and not at all cruel, for the law to demand that Eliot Spitzer forswear sexual relations with a high-class hooker.
Or so I would argue. Obviously, these sort of judgments are prudential rather than definitive, and I'm not sure where I would draw the line in many instances. And one appeal of setting a "consenting adults" standard on morals legislation, rather than the more latitudinarian standards I've suggested, is that it removes at least some of the subjective element from these debates. But I don't have really have a problem with a standard for morals legislation that leaves vast gray areas where people can argue back and forth: That's why we have the democratic process.