Besides exposing his own profound ignorance of both Holy Scripture and Christian tradition regarding human sexuality, Meacham makes it clear that his agenda isn't organic change but revolution. Daniel Larison makes a better response to Meacham's error-filled diatribe than I could:
Having already shown that he has no grasp of [Christian tradition or scripture regarding human sexuality], Meacham proceeds with his “Christian case for gay marriage.” He puts enormous weight on the intrinsic nature of homosexuality, which is to make a quality of postlapsarian nature normative. In a fallen world, everyone has a predisposition to act contrary to our true nature, but in no other case that I can think of do we pretend that indulging such a predisposition is inevitable, much less something to be embraced and approved. Meachem is no more persuasive or credible when he cites examples of how certain passages have been abused in the past. Nowhere in his article does Meacham even begin to take seriously the central importance of denying oneself in Christian discipleship. God did not call His people to indulge their inclinations, but to deny themselves to follow Him. This is why the comparison with race is so inapt and ultimately so absurd. There is no way that, and no reason why, someone of any race could refrain from being the way he was born. Homosexuality is entirely different, in that acting on it is a matter of volition and a determination to pursue one’s own will rather than denying it. Whether or not one is born with such an inclination, that would not be a license to indulge that inclination. Meacham’s argument is essentialist and actually denies the responsibility and agency of homosexuals, which is far more of an attack on their humanity than refusing to allow them to “marry.”
The heart of Meacham’s argument does not bear much scrutiny, and we have not even come to the question of how entirely divorced Meacham’s entire argument is from a Christian understanding of the purpose of marriage. Procreation is an important part of that purpose, and joining two people from different sexes in complementary relationship is another, but beyond that it is a vocation to unite oneself to a person radically different from oneself. The uniting of complementary opposites as a type of the unity between Christ and His Church is one of the mystical meanings of marriage. The Christian conception of marriage is of two people joined into one flesh, the full expression of which is a child. Nowhere in the “great Judeo-Christian tradition” that Meacham supposedly takes so seriously is there support for his argument.