Augustine believed that prelapsarian erections happened not out of passion, but direct rational volition, like choosing to raise your arm. (To save you from what you might see if you Google "prelapsarian erections," that is a phrase I actually came across in a theology textbook that refers to erections before the Fall.) Sexual passion as such is sinful for Augustine, and this view would dominate the Christian tradition.
Jovinian was declared a heretic in the fourth-century for claiming that marriage was morally on par with celibacy.
Aquinas taught that masturbation was a more serious sin than rape, because at least with rape procreation could happen. (Impeccable logic, right?)
Throughout much of medieval European Christianity, couples were advised by priests to refrain from sex during Lent, Advent, on Friday's because that is when Jesus was killed, on Sundays because that is when he rose, and many other special days as well.
Like I said, we've had a hard time being very positive about sex. The dominant tradition has held that sex should be as rare as possible and only for making babies. Celibacy is the best, but if you can't go with that, get married and have as little sex as possible and try not to enjoy it. It isn't that the Bible and Christian tradition are totally bereft of resources for a positive view of sexuality. Many passages in the biblical erotic poem Song of Songs (or Song of Solomon), for example, are enough to make many of us blush. (Interesting side note: the most copied and commented on book of the Bible throughout the Middle Ages is the Song of Songs. One can only imagine why celibate monks were drawn to this book over and over again.) But even though there are biblical resources for a high view of sexual pleasure, these became quickly lost in a tradition that increasingly was influenced by philosophical views (such as Neo-Platonism and Stoicism) that lead Christians to locate the spiritual life in rational detachment from bodily pleasures or pains.
But as much as we have gotten wrong, one thing our tradition has gotten right is that sexual energy is powerful and is filled with potential, not only for great joy, but also for profound harm and destruction. There certainly need to be certain relational parameters around sex, such as commitment, mutuality, equality, and so on, and yet our culture at large seems to willfully ignore the high costs of sexual expression outside of deep relational bonds. As Rob Bell put it in his book Sex God, the church has made the mistake of thinking that human beings are supposed to be angels, while our culture has made the mistake of thinking that we are mere animals.
I know that many people see the gay marriage debate, and the emerging Christian reevaluation of same-sex relationships, as the biggest shift ever in Christian sexual ethics, but I think that is a mistake. I think the biggest shift occurred in the middle of the last century as many Christian denominations started morally approving of contraception, and thereby affirming that sexual pleasure and intimate bonding are morally legitimate goods of sexual expression, apart from the desire or capacity to reproduce. The vast majority of Christian tradition, again, has condemned nonprocreative sex for heterosexual couples. For many churches to shift away from this tradition in the last century was an enormous change that shouldn't be underestimated, and its significance for the gay marriage debate shouldn't be overlooked.
I agree with this shift, and think that one of the greatest tasks of the church today is to reformulate our sexual ethics in such a way as to celebrate this good gift of our Creator for what it is. That said, it seems to me that approval of gay marriage is a natural and consistent extension of the sexual ethics embraced by most Christian denominations today.
If sex can be celebrated and affirmed as something that is good for opposite-sex couples, not just for reproduction, but for mutual pleasure and intimate bonding, then what is our basis for denying moral approval to sexual intimacy between same-sex couples?
In studying the history of sexual ethics in the Christian tradition, and specifically just how much even current "traditional" or "conservative" views break with the dominant view of church tradition, I realized that it is grossly misleading to frame the current debate as those Christians who are in favor of "traditional" marriage versus those Christians who want to make a radical break with tradition. Instead, it seems much more accurate to describe the debate in terms of Christians who want to stop the radical reformulation of sexual ethics begun in the 20th century with heterosexual couples, and those Christians who think that extending this reformulation to homosexual couples is the right thing to do. Most conservative Christians have let go of the primary traditional reason for an absolute prohibition on same-sex relationships (that being that all nonprocreative sex is immoral), yet still hold to the prohibition. This is a problem.
Even though I disagree strongly with the official Roman Catholic view of sexuality, which still sees procreative possibility as necessary for any sexual relationship to be legitimate, I also respect it for its consistency. Within the Roman Catholic sexual ethical framework, it is still consistent to condemn all same-sex relationships. It is, I believe, a wrong framework with internal consistency. This consistency is lacking in most Protestant ethical frameworks that no longer require procreative possibility for heterosexual couples. We have the right framework, I believe, but deep internal inconsistency.
Personally, one of the main reasons I changed my mind about gay marriage, aside from my growing sense that the traditional heterosexist interpretations of Scripture are not near as strong as I once thought they were, is because I realized I was being inconsistent in the beliefs I held about straight marriage. I believe, like most Christians I know, that marriage between a man and a woman is essentially the deepest of all friendships, where two people completely give themselves to one another wholeheartedly and unreservedly, and this may or may not involve having children. I wouldn't want to diminish the importance of having and raising children in marriage, but at the same time, I can see nothing that would require married couples to try to have children. Certainly our current marriage laws make no such requirement. But when it came to discussions about gay marriage, I would revert back to arguments about reproductive complementarity and such, not realizing for a long time the intellectual hypocrisy in that move.
When it comes to our moral views, we cannot have certainty, granted, but we can at least strive for consistency. Jesus got cranked up the most about hypocrisy, which is a moral condition of being internally divided. Intellectual inconsistency is the cousin of hypocrisy. It divides our minds into applying different sets of criteria to positions we are not inclined to embrace than to our own.
So I changed my mind, in large part, because I thought that embracing the same moral parameters for homosexual relationships as heterosexual relationships makes the most sense- it is the most coherent and consistent view. I will say it wasn't easy for me. I have a very conservative temperament, and any intellectual shift I have had has not been without countless hours of study and countless hours of sleepless nights. I kept thinking to myself, "But there is just something morally significant about it being a man and woman, and not a man and a man or a woman and woman." But through lots of soul-searching and reflection, I just couldn't name exactly what that supposed morally relevant difference is.
There is a story about C.S. Lewis as a boy telling his father that he was prejudiced against the French, and his dad asked him, "Why?" Lewis responded, "If I knew why, it wouldn't be a prejudice, would it?"
I could no longer say why a committed and loving same-sex couple was living in sin, so I decided it must be a prejudice I should let go of. So I did.