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 don’t 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” (that’s 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” (that’s 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” (that’s the well-known reactionary Bertrand Russell).
Nor, perhaps more importantly, is it difficult to find various traditional features of marriage law that only make sense given the procreative understanding: For instance, the granting, not of divorces, but annulments in the case of marriages that weren’t or couldn’t be consummated — a provision with deep roots in the common law tradition, and one that remains in force today in contexts as diverse as California and England. (Current English annulment law went on the books all the way back in the dark medieval year of … 1973.)
Note, too, that by saying that a marriage left unconsummated through coitus is invalid, the common-law tradition makes precisely the distinction that Drum (and many others) find so self-evidently ridiculous and assume was obviously just invented for the gay marriage debate — a distinction between relationships that involve the reproductive act and those that don’t, with the former being valid marriages even when they’re infertile and the latter not. This Robert George-esque view of what is and isn’t marriage may or may not make sense, but it was considered a perfectly reasonable way of drawing distinctions between heterosexual relationships long before the homosexual claim to equal marriage rights began to be advanced. Wise or not, it was a distinction inherited from centuries of legal tradition, not invented as a made-up way to keep the gays from contaminating marriage.