In discussions about same-sex marriage within the church, there is a common argumentative strategy on the part of progressives to acknowledge that the Bible indeed says some negative stuff about same-sex relationships, but they will then try to diffuse the significance of this by going on to ask the rhetorical question, "What about slaves and women?" Just as people in the past used (or, as most of us would agree, "misused") the Bible and church tradition to justify pro-slavery and anti-women attitudes and practices, so now people misuse the Bible and church tradition to stigmatize and exclude gay and lesbian persons from full inclusion in church and society.
As I have written before, I think the analogies of women and slaves are helpful in some ways, but the analogies only go so far. They don't show that since we were wrong about those issues, we must therefore be wrong about this one. They just show that we might be wrong about this issue, since we have gotten things wrong in a major way in the past. These two analogies show that longstanding church tradition and a handful of biblical quotes do not suffice to guarantee one's position is authentically Christian, but, again, much more has to be said by the progressive Christian to make their case.
It seems to me that in this discussion and debate over the consecration of same-sex relationships, the church would be better served by focusing less on the analogies of women and slaves, and more on the analogies of celibacy and contraception. These two issues not only represent ways in which the church has radically changed its position, but since these two issues are from the realm of sexual ethics, they seem much more intrinsically connected to the issue of same-sex relationships.
Let's start with the issue of celibacy. While the orthodox church has always taught that marriage is good (primarily for procreative purposes), up until the Reformation era about 500 years ago it also taught that celibacy and virginity are morally and spiritually better. In fact, as a 4th century theologian named Jovinian learned the hard way, you could be declared a heretic for suggesting that marriage is just as good as celibacy.
When you put yourself in the position of the Reformers, and you make the argument that clergy don't have to be celibate because marriage shares an equal moral status with virginity, you are making an argument that goes against the church's way of interpreting the teachings of Jesus (Matt 19:12, Luke 20:27-40) and Paul (1 Cor 7) for nearly 1500 years. But the Reformers did it, and now we Protestant Christians take it for granted. What was essentially the most radical shift in Christian sexual ethics up until that point we would now yawn over if we heard someone debate it.
Fast forward another few hundred years to 1930, the year marking the first time any church denomination officially authorized the use of contraception. When the Anglican church made this unprecedented move, they were going against nearly two millennia of church tradition that strongly and universally condemned contraception. This is hard for us to appreciate now in this day and age, but the Christian condemnation of contraception was just as severe throughout history as was its condemnation of same-sex relationships. Although marriage had always been considered good in the church, marital sex, by and large, was only seen as morally good if it was for the purpose of procreation. Marital sex only for pleasure was seen as lustful and even "against nature." Therefore all contraceptive or intentionally non-procreative sex was seen as a grave sin.
Just as Christians could (and still do) appeal to the story of God's destruction of Sodom (Gen 19) to justify God's supposed abhorrence of all same-sex relationships, Christians could appeal to the story of God's killing of Onan (Gen 38) to justify God's supposed abhorrence of all contraception.
Defenders of contraception would want to argue that Onan was killed, not for contraception as such, but for selfishly refusing to honor his brother by agreeing to follow the custom of levirate marriage (levir is the Latin word for "brother-in law"), whereby if a man dies without having a son, his brother is obligated to try to reproduce with his widow so that a male offspring can be reckoned as his and can carry on the family name and control the land. This argument, though, doesn't sit well with the fact that the punishment for refusing to do this was public shaming, not death (Deut. 25:5-10), and in the history of the church there has been, up until 1930, a universal condemnation of contraception with this passage being used most frequently to show that God is against all intentionally non-procreative sex. While it is hard to say with certainty that the "original meaning" of the Onan story was a condemnation of all contraception (although a strong argument could be made that it was), we can say with certainty that this is how the church throughout history and across the globe interpreted this passage for nearly two thousand years. Yet now, even for the vast majority of Roman Catholics whose church hierarchy still forbids artificial contraceptive methods, contraception is not even seen as a live moral debate. In a period of less than half a century, contraception went from being universally condemned to being nearly universally embraced.
It seems to me that the ethical issue of contraception is the most insightful analogy for thinking through the church's ethical debates about same-sex marriage. Not only does it show a major reversal of a traditional church teaching, but the reasons for this reversal are deeply connected to the current movement among many to embrace same-sex marriage in the church. Contraception became permissible as the church's understanding of marital sex shifted to focus more on intimacy and pleasure, not just procreation. Once the church accepts that marital sex is good based on the values of intimate bonding and mutual pleasure, without procreative intent or possibility required, then it becomes much harder to say exactly what could be wrong with two people of the same sex living in a covenant relationship and basing their sexual lives together on these values as well.
While I understand the debate over same-sex marriage cannot be reduced to these considerations I have raised, in my experience those who are against same-sex marriage place a great deal of rhetorical weight on the "two thousand years" argument in their case. Its rhetorical power, however, masks its logical weakness and historical naivete.
"Who are we to question the universal and historic church on this issue?" they will say. Well, we are part of a church that has a long history of challenging what came before it. We are part of a church that has changed its mind about many things, including the value and purpose of sex. We are part of a church that is always in need of reform. We are part of a church that has been growing and changing for a good while now. We are part of a church that has made a tradition out of questioning tradition. That's who we are.