I'm no expert, but I believe many, if not most ceremonies involving sexuality were either tributes to gods and goddesses of fertility, or rites that were supposed to facilitate conception/childbirth.

When judeo-christian religions took over.

I don't know how much of the loss of believers is due to a lack of sex, but it surely contributes to the problem of a lack of priests/nuns that the catholics are having these days.

Many sexual beings don't have spiritual needs. I think the sexually-repressed nature of christianity is part of a bigger problem the religion faces today, that problem being how out of touch it is with modern society. And why shouldn't a religion be out of touch in 2007?

Paranoid, my somewhat educated theory on the matter is even more bleak, I'm afraid. For what it's worth, I do have some background in sociology and psychology, so this isn't a blind guess, exactly.

I don't see significant loss of membership due to denial of sexuality. The opposite seems to be true. As you're well aware, sexual problems, particularly those acquired during pre-adolescence and adolescence, tend to last our entire lifetime. What you get when you combine Christianity (complete, blind belief in absolute truth) with the notion that sexuality is dirty or bad (childhood abuse, if you ask me, to tell that sort of thing to children) you get people who are going to live their lives by the idea that there's something wrong with them.

And when they have kids, they're going to teach the same things to them...

I think not only is the problem bad for Christians, it's even worse, because even after people leave Christianity, it's very, very difficult for them to disabuse themselves of many of their notions of sex. It's just such a personal, illogical, and private issue!

So, I'd suggest that in some ways, it actually keeps people in Christianity. If you really believe it's bad, and the church says you have to stay in the church to have any hope of defeating your sexuality, then that's one more chain that's going to hold you.

Furthermore, it stays in the psyche of those who leave religion, so that even non-Christians often have screwed up notions about sexuality from their past, after they've left the religion.

Boy, I feel down now. That's just depressing to think about.

At the time of Christianity's founding, the mainstream Roman culture embraced fertility ceremonies.  Seriously, wedding cakes of the era would have been considered totally inappropriate for children by modern standards.

My theory is that, because Christianity was part of the counterculture, it took on a very anti-sex, ascetic character from the very beginning.  This may not be my theory alone--probably somebody else with more supporting evidence thought of it first.  It's just the overall impression I get from my reading so far.

I do know that Christianity had a very early, unfortunate association with ascetic  trends in both the Jewish and Greek traditions, the latter growing out of the dualistic matter=evil, spirit=good prejudice of the post-Zoroastrians.  Earlier I mentioned Augustin of Hippo as a concrete example of this influence on early Christianity, and his influence on the Catholics in particular is immeasurable.   Check out his_Confessions_ for the full (contrived IMO) rationale for why the body is evil and the spirit is good.