I believe some of the earliest religions were essentially sex cults that attempted to appease the gods and goddesses of fertility. More generally, the first shamans and priests were those who had acquired some insight into the workings of the natural world, yet, intentionally or not, shrouded them in mystique and ritual, creating barriers of entry to the knowledge they had discovered.

That sex is a particularly strong human motivator would drives religion's appeal to religion consumers while sex's role as a status differentiator would drive the appeal to religion producers. In short, I think the role of sex in religion is less a recent growth strategy than an ancient component that helps explain the religions we have today.