If gay marriage opponents had essentially invented a procreative foundation for marriage in order to justify opposing same-sex wedlock, it would indeed be telling evidence of a movement groping for reasons to justify its bigotry. But of course that essential connection was assumed in Western law and culture long before gay marriage emerged as a controversy or a cause. You dont have to look very hard to find quotes (like the ones collected in this Heritage Foundation brief) from jurists, scholars, anthropologists and others, writing in historical contexts entirely removed from the gay marriage debate, making the case that the first purpose of matrimony, by the laws of nature and society, is procreation (thats a California Supreme Court ruling in 1859), describing the institution of marriage as one founded in nature, but modified by civil society: the one directing man to continue and multiply his species, the other prescribing the manner in which that natural impulse must be confined and regulated (thats William Blackstone), and acknowledging that it is through children alone that sexual relations become important to society, and worthy to be taken cognizance of by a legal institution (thats the well-known reactionary Bertrand Russell).