The arguments against the federal legalization of prostitution can be generalized into two distinct but complementary classes; those that seek to establish a causal connection between it and some undesirable consequence or another, and those that simply contend in various ways that personal ethical objections to it are sufficient for prohibition. As intuitive as they might initially appear, however, neither class should be convincing to anyone with more than a passing interest in sensible democracy.
Every argument from the first class can be classified as possibly dubious and certainly vacuous, regardless of its specific content. Some argue that prostitution serves as an impetus to slavery, some argue that it promotes criminal behavior, and some argue that it leads to a greater incidence of sexually-transmitted infections, but even if these are true, not only has prohibition not effectively solved the problem, in removing prostitution from under the aegis of regulation it may have actually exacerbated it. (More sophisticated opponents respond by smuggling into the argument even subtler concepts like wage slavery and familial dysfunction, but those moves in this class just continue to get progressively slipperier.)
The arguments of the second class are the far more dangerous, in any case, because rather than present neat, falsifiable claims about consequences, they instead rely on cherry-picked abstractions and loaded emotional appeals. Our natural (and probably correct) intuitions that prostitution is demeaning to women or that it is guilty of degenerating sexual culture can lead us to favor prohibitions of a categorical sort, even when such action is ultimately self-defeating. (We find ourselves in the same kind of conundrum when justifying silencing obnoxious groups like the Klan and the Westboro Baptist Church.) Some of the arguments in this class can be quite superficially compelling, too. For instance, it seems at least plausible that even if the practice of prostitution is not itself immediately assumed to be immoral in some objective sense, its attraction is due primarily to its taboo, and it thus can be taken as a kind of antisocial deviance. This is not sufficient cause for prohibition, but it's certainly suggestive of it, and it frames the grounds on which the opponents must in turn frame the issue: the law doesn't track the precepts of morality, it tracks the norms of pragmatism.
But the argument against legalization fails even on pragmatic grounds, because it suffers an inconvenient counterexample: all the relevant problems with regulated prostitution arise in identical form during the creation of pornography, which similarly commodifies sex and raises similar regulatory concerns but is able to take sanctuary in the First Amendment. Outside of this consideration (and apparently, according to California v. Freeman, the arcana of the payment scheme), there is no legitimate distinction between them, and thus, a opponent of prostitution is left in the awkward position of having to explain why commodified sex prearranged for broadcast should somehow be endowed with a legal permissibility denied to ordinary prostitution. Legal pornography more than just necessitates legal prostitution—it analytically entails it. The opponents got this right—and, I concede, they have from the beginning—but unfortunately for them, we can't sensibly ban pornography.
You might have noticed, I omitted an old standard; perhaps its merely a matter of semantic squeamishness, but it seems crass to label a crime “victimless” when at every commission, there are at least two victims. Does this change our intuitions about the subject? It shouldn't. Rather, it ought to prompt us to realize that as usual, we've stressed the trivial and left the profound unanswered; rather than ask “How do we fight prostitution?” we ought to have been asking “Why has there emerged a market for victims in the first place?"
