Commercial surrogacy might look like the harnessing of technology to enhance reproductive freedom. But in reality, this practice invariably involves wealthy couples renting poorer women's bodies. And that is not something leftists ought to support.

I don't want to minimize the enormous emotional challenges posed by the unrequited desire to have a child of one's own. But it is that "of one's own" that is the crucial qualifier in any discussion of surrogacy. There is no shortage of children who could benefit from being adopted. America's foster care system is overflowing. Impoverished nations cannot feed their growing populations. There are more than enough kids to go around. The demand for surrogacy is not about making families; rather, it's about the very specific desire to rear from birth a child containing genetic material from at least one member of the couple. And fulfilling that desire through surrogacy requires the use of another human being whose ability to consent is hindered by her relative poverty and powerlessness.

Commercial surrogacy poses a real danger to the principles of economic justice that are at the core of progressive values. And that danger outweighs the supposed benefits of a practice that, despite its emotional pull, is best considered a luxury. Indeed, commercial surrogacy represents the final conquest of the consumerist logic of capitalism, turning women and children into commodities whose value is determined by the market.

Leftists have long argued that the morality of an economic arrangement cannot rest solely on the apparent consent of the parties to a contract. Even if a man consents to sweatshop wages, he is probably doing so out of an economic desperation that limits his freedom in a way not felt by his employer. Therefore, progressives argue, some contracts are immoral — namely those that exploit the powerlessness of the underprivileged. It is the role of government, then, to make such contracts unenforceable, and in so doing to equalize as much as possible the unfairness that can rend society.

Commercial surrogacy arrangements are just the sort of contracts that the left usually rightly condemns. It is almost always a wealthy couple renting the womb of a woman who is less well-off. From start to finish, the process of assisted reproduction by surrogate will set you back $80,000 to $120,000 — up to twice the American median household income. And on the other side, consider this: When New York Times reporter Alex Kuczynski perused the catalog of "gestational carriers" for her surrogacy, none of the candidates had household incomes greater than $50,000. The $25,000 fee for a low-income surrogate is quite a windfall.

Now, some progressives might argue that low-income women should be free to use their bodies to generate that sort of windfall. But there's another wrinkle. Kuczynski revealed that very poor women are not generally surrogates — not because they don't want to be, but because surrogacy agencies (and, by extension, their clients) don't want them to be. Instead, there's a sweet spot in the lower middle class in which the women are well-off enough to be desirable surrogates, but not well-off enough that renting their bodies to rich people is off the table.

As the demand for surrogacy expands (it has grown over 250 percent in Britain, where commercial surrogacy is not even legal, over the past six years), the share of enthusiastic, financially secure surrogates will dwindle. And that trend line raises a key question: What if legal commercial surrogacy were to become the standard across the U.S.?

First, of course, would be an explosion of the practice among traditional surrogate-seeking groups: wealthy and infertile opposite-sex couples and wealthy same-sex couples. And going beyond that, widespread commercial surrogacy would permit the institutionalized outsourcing of the physical and affective labor of pregnancy from the wealthy and powerful to the underprivileged and powerless. It is the ultimate conquest of consumer capitalism: persons themselves.

Imagine a drab storefront in a wealthy suburban enclave. Inside there are two rooms, each with walls full of screens. In the first, the screens display information about the qualities of gamete donors, such as intelligence, attractiveness, and race. Here, some customers bid on the gamete(s) they need for the child they envision. In the next room information about surrogate options is displayed, graded on criteria such as overall health, social stability, and psychological reliability. After securing a surrogate, the happy customers leave to prepare the nursery.

The only distinctions between the image I'm proposing and what already happens are scale, efficiency, and honesty. However it works in practice, commercial surrogacy entails letting the market determine prices for the creation of human beings and for the leasing of women's bodies. (Pregnancy is not glorified babysitting, and no one who has been pregnant or been close to a pregnant woman could believe it is; pregnancy involves the whole person, body and mind, at every conscious and unconscious moment for nine months.) This is the final victory of the logic of the market and of capitalism itself: the act of procreation — the primal behavior of human beings — transmuted by the market into a consumer behavior from which profit can be extracted.

There are two primary arguments a sincere leftist could mount in defense of commercial surrogacy: the argument from autonomy and the argument from equality. The former, which objects that banning commercial surrogacy attacks the autonomy of women to do what they like with their bodies, has largely been asked and answered. Commercial surrogacy invariably involves wealth and power disparities that vitiate at least in part the autonomy of the surrogate. This objection really smacks more of Lochner v. New York-style libertarianism than anything else. Like all economic libertarianism, in practice it amounts to favoring the privileged over the poor.

The argument from equality, specifically LGBT equality, goes like this: If marriage equality is to mean something comprehensive, it must include, as much as possible, access to the multifarious experiences of family life open to opposite-sex couples. One of the most profound of these experiences is the raising from birth of a child containing genetic material from at least one partner.

The question then becomes: What do you value more? If you value this aspect of LGBT equality more than the fundamental issues of social and economic justice undercut by commercial surrogacy, then you should support commercial surrogacy. But then what is left of the left? Stripped of any substantive attachment to economic justice, all that remains is libertarianism gussied up with gauzy rhetoric about the poor and the marginalized.

But that's not the left that I'm writing to, that I respect, and that we need. We need a faction standing up loudly and comprehensively for the poor, the exploited, the underprivileged. And we need that faction to be willing to stand up against tempting innovations like commercial surrogacy that further widen the chasm between the powerful and the powerless — and that extend the oppressive logic of the market to its farthest and final frontier.