As I indicated back in August 2005, when Bates first propounded his argument, the logically pivotal issue is whether the eschatological union between Christ and the Church as a whole is transferable to that between Christ and individuals, in such a way that homosexual marriage between Christians would be just as effectual a sacrament of it as heterosexual marriage between Christians. That's the core of Bates' argument, which is really a summary of what he calls The Episcopal Church's argument. Having followed the Bates-Brandon debate about it, I am confirmed in my original intuition that the only way to block the inference Bates wants to make is to invoke, on independent grounds, an understanding of human sexuality on which homosexual acts are intrinsically immoral and thus homosexuality, defined as the inclination to perform such acts, is objectively disordered. Bates himself suggests two readings that present a decent case for that: Cardinal Angelo Scola's The Nuptial Mystery and Pope John Paul II's Man and Woman He Created Them. For those lacking either the time or the money for such books, I also suggest William May's article "On The Impossibility of Same-Sex Marriage."
What such sources do is present arguments, based on Scripture, Tradition, and natural law, that the "unitive" significance of sexual intercourse cannot be had without procreative significance. If that is so, then there can be no such thing as same-sex marriage, and a fortiori no sacrament thereof. That serves of course as an argument against contraception too, which is no accident. Once the ancient consensus about contraception is abandoned, as the Anglican Communion did in 1930, the jig is up. The same, I would add, goes for the women's-ordination issue. But that's a topic for another post.