There are two other reasons for the current unprecedented advocacy of gay marriage. The first is the decline of any public recognition of sexual difference and so the significance of sexually asymmetric unions, which I’ve already alluded to. The second, and arguably most important factor, is the technologisation of childbirth, allied to the increased acceptance of the adoption of children by gay couples.
Since the link between sex and childbirth is becoming increasingly tenuous, heterosexual marriage is increasingly connected with child-rearing rather than with procreation. In which case, indeed, why should not gay couples sustain the same connection with an equal capacity?
Are these reasons good reasons? It is notable that even the churches do not seem to dare to address the first issue of sexual difference, despite the fact that they recognise the validity of childless heterosexual marriage, that they have in modern times increasingly stressed mutual affection as one of the goods of the married state, and that both Augustine and Aquinas regarded marriage between man and woman as the most intimate mode of specifically natural human friendship.
-John Millbank (source)
If the Christian churches cannot come up with a balanced, biblical, and compelling account of the complimentary nature of men and women as well as some deeper answer to the issue of childbirth (which has become, basically, a consumer service) then it will have not reason to stand in opposition to gay marriage. To date, I am seeing little to give me hope that these issues will be helpfully addressed by Christians.