The authors believe that there is a Platonic form of marriage that is independent of any social or legal circumstances, consisting of the union of one man and one woman, sexually exclusive and long-lasting if not eternal. This is a species of natural law myth-making, less entertaining but scarcely more plausible than the tale of primordially conjoined male-male, female-female, and male-female creatures that Plato puts into Aristophanes' mouth in the Symposium. That famous myth imagines a world of heterosexuals, gay men, and lesbians, each seeking its original other half.
George & Co. see Nature as authenticating only heterosexual unions. They ground this belief in body parts and functions: Because only one man and woman can engage in coitus at one time, and coitus is the only (natural) way of producing offspring to continue the human race, then "real marriage" can only consist in a male-female duo. Any other marital arrangement is not the real thing.
Societies make their rules according to their own evolving values--based on nature, custom, religious teaching, calculations of social utility, individual rights claims, and the politics involved in sorting out all of the above. Whatever merits the Georgian "conjugal view" of marriage may possess, nothing qualifies it as a timeless truth.
There was an interesting discussion of polygamy in the ellipses. I sympathize more with Silk's view of marriage than George's. George's natural law view does require an "input" (his Platonic myth) to end up with his "output" of conjugal marriage as the exclusive "natural" form of marriage. But an observation of nature just as much validates using what "Plato puts into Aristophanes' mouth in the Symposium" as our input which leads to a gay marriage output.