A Dahlia Lithwick article posted a couple of weeks ago on Slate was dismissive of the case against gay marriage based upon the metaphor of a "slippery slope". A slippery slope argument tries to show that, in some cases, once a concession is granted to one's opponent there is no logical basis for stopping with just that single concession. Consistency demands that the next point of contention also be conceded, and so on, until one finds oneself slipping helplessly toward an unacceptable endpoint, unable to gain a firm handhold anywhere along the way to stop the slide.
As regards the issue of gay marriage the slippery slope is invoked to argue that once marriage is no longer limited to a man and a woman we have no compelling reason to stop at restricting marriage to just two people of any gender. Pressure would doubtless arise to extend marriage rights to any combination of people who consent to be married, no matter the number, gender, or biological relationship of the intendeds, and the courts would have no basis for opposing the wishes of plaintiffs who would almost certainly challenge laws against such unions.
Lithwick is having none of this sort of reasoning, however. After spending much time ridiculing those who fear that redefining marriage may ultimately lead to state sanction of conjugal relationships between humans and their pets and other such perversities, she addresses the more immediate and plausible concern that marriage rights will eventually be extended to groups of people. Lithwick doesn't see this as a likely consequence of allowing gays to marry. Her reason is that there is a rational basis for states to ban polygamous and polyamorous marriages because historically these have resulted in imbalances of power, coercion (particularly of young girls), and an enormous financial burden to the state.
But these seem to be tenuous grounds for resisting state approval of polygamous unions, since the first two are not necessarily inherent in those unions, and the third, despite Lithwick's denial in her article, would make polygamous unions no different than gay couplings (See here, for example). If the liabilities she mentions were removed or mitigated, what grounds would remain for states to proscribe such relationships? Lithwick's argument seems to entail that if these reasons were to be found unpersuasive by the courts then polygamy would indeed be legally inevitable.
She does mention one additional reason for pooh-poohing the slippery slope argument. She notes that the constitutional right to privacy, which some justices imagined they descried in the Griswold case of 1965 and upon which they based their Roe decision of 1973, is an intimate right, between two consenting partners.The desire of a group of seven people to marry simply does not intuitively fit into that binary sphere of intimacy.
In other words, extending marriage rights to two gays is just not the same as extending similar rights to multiple partners because the latter combination is not intuitively intimate whereas relationships between couples are. Courts may, then, on the basis of intuitive intimacy, refuse to polygamists what they grant to homosexuals. How does Lithwick propose, however, that courts assess the degree of potential intimacy residing in a particular group of would-be spouses? Would the criterion of intuitive intimacy, in the interest of equal treatment under the law, be extended to heterosexual couples who appear to some official to be insufficiently affectionate toward each other? Do we want judges and legislators making decisions about matters of law on the basis of their personal understanding of intimacy?
It seems pretty clear that, pace Lithwick, once gender is no longer relevant to marriage there will be no compelling reason to keep the number of people involved relevant either. Whether this would mean the end of marriage or not and what the long term social effects would be, who can say, but it would certainly be another body blow to an institution that has been severely rocked by liberalized divorce laws, liberalized views on cohabitation, and liberalized sexual mores over the last thirty five years.