In some ways the title of this post is misleading: I'm not discussing the reaction of late antique Christian writers to same-sex marriage (there are rare examples of such marriages in Roman times: see Mathew Kuefler, "The marriage revolution in late antiquity: the Theodosian Code and later Roman marriage law", Journal of Family History 32 (2007), p. 363). Instead, what I'm interested in is how the arguments of (some) Christians in Britain today on same-sex marriage are drawing on ideas of marriage developed in the few five centuries of Christianity, rather than directly from the Bible itself, and yet ignoring elements of this "traditional" view that do not suit them.
One of the key arguments repeated in opposition to same-sex marriage is that marriage is intended for the procreation of children and that therefore any relationship in which procreation between the couple is impossible cannot be regarded as marriage. This is often extended in Catholic teaching to the claim that any sexual activity between a couple that is non-procreative is sinful.
However, the Bible says nothing explicitly about contraception (although there have been attempts to claim that the punishment of Onan in Genesis 38: 8-10 shows that all contraceptive acts are wrong). While the Creation account in Genesis 1 has God telling humans to increase and multiply, Genesis 2 has Eve created as Adam's companion and there is no mention of marriage as involving intercourse or bearing children before the Fall.
Overall, however, the Old Testament sees marriage and procreation as good and blessings from God; the New Testament is far more ambivalent towards both. In particular, St Paul's discussions of marriage (the most extensive in the New Testament) have almost nothing to say about procreation: when he says that it is "better to marry than to burn", there's no suggestion that this is only for those willing and able to have children.
The view that sexual activity is only morally acceptable for the procreation of children comes not from the Bible, but from Stoic philosophy, but it was taken up both by early Christian Fathers such as Clement of Alexandria and slightly later ones, such as Jerome in the fourth century. As John T. Noonan puts it in Contraception: a history of its treatment by the Catholic theologians and canonists (Harvard UP, 1965), p. 49: "The statement that the rational use of sexual faculties is a procreative one is not the same as the Old Testament statement that fecundity is highly desirable. The ideas do, however, harmonize".
As discussed in detail both by Noonan and by Peter Brown, The body and society: men, women, and sexual renunciation in early Christianity (Columbia UP, 1988), a developed theology of marriage and its purposes, embodied in the works of St Augustine, took place in the late fourth and early fifth century against a background of intellectual challenges from ascetics and sects regarded as heretical. Augustine attempted to assert the good of marriage against opponents who mainly saw it as vastly inferior to virginity and sexual abstention. Some, like Jerome, asserted that the only good thing about marriage was that it generated children who could become virgins. The Manichees, meanwhile, saw procreative sex as particularly evil, generating children who were caught in the body and the darkness of the material world. Some of them, at least, argued for efforts to ensure that any sexual activity was non-procreative. On the other side of the debate were Christian writers such as Jovinian, who saw marriage and virginity as equally good.
As Kate Cooper and Conrad Leyser have discussed, this whole debate was also implicitly about authority within the church and within society. Renunciation of sex (specifically male renunciation of the sexual use of women) had become a symbol of the wider self-control that a man in authority needed to have, and a symbol that was particularly promoted by ascetics from a lower social position anxious to prove their moral superiority. This was a challenge to the other obvious claimants to Christian leadership in late antique society, the couple of senatorial social status.
Augustine attempted to steer a middle course, opposing the condemnation of marriage as heretical, but wanting to remain within a tradition that saw virginity as superior. The result was that he produced a distinctly ambivalent defence of marriage which described all sexual pleasure as sinful. It's also typical of his own limits of perspective that he argued that Adam and Eve must have been intended by God to have intercourse in Eden: if Adam had just wanted friendship, a man would have been a more stimulating companion (Brown p. 402).
Tangled up with this was another near-simultaneous debate about the virginity of Mary, discussed in most detail by David G. Hunter, Marriage, celibacy, and heresy in ancient Christianity: the Jovinianist controversy (Oxford UP, 2007). Had Mary remained a virgin while giving birth to Jesus (virginitas in partu), i.e. had God allowed some kind of miraculous painless birth, as various apocryphal Gospels claimed? Had she remained a virgin after giving birth (virginitas post partum), i.e. had she and Joseph been in a permanently celibate marriage? Neither were agreed doctrines in the fourth century. Ambrose of Milan, however, anxious to protect the consecrated virgins of his church from pressure to marry, and seeing Mary as a symbol both of such a virginal life and of the inviolable church, became the most influential supporter of Mary's perpetual virginity during birth and after.
This was the view of Mary that triumphed in the western church. However, it was also important to maintain that Mary and Joseph had been truly married, for obvious reasons. As a result of these debates, the doctrine that Augustine developed about the purposes of marriage sees multiple "goods" in it: offspring (proles), fidelity (fides) and symbolic stability (sacramentum, which didn't yet mean "sacrament" in the Catholic sense). The sacramentum, in the sense of an indissoluble bond, meant that a couple who could not have children together could not divorce so that one might remarry and have children. The procreational good of marriage could be dispensed with. This could be either on grounds of incapacity (the old could marry for mutual companionship) or on grounds of choice (continence in marriage was preferable to intercourse).
Reflecting this view, the medieval church almost always adhered to the view that it was consent, not consummation that made a marriage. The churches' current opposition to same-sex marriage on the basis that the nature of Christian marriage requires the procreation of children turns out to be based on extremely shaky historical and theological foundations.