Sex and spirituality are majorly linked in Stranger in a Strange Land. When Mike first comes to Earth, he uses water as a means to bond with people and form "water brothers" (more on water in the "Symbols, Imagery, Allegory" section). But eventually he discovers sex, as any young man is bound to do. And just like that, sex replaces water.
Mike links sex with his personal spiritual proverb, "Thou Art God" (24.163). But for Mike, sex isn't something that should be kept solely between two people in a traditional marriage. If sex is a way to understand God, and everyone is a part of God, then the more we express ourselves through sex— healthy sex, mind you—the more we understand about the nature of God. This is why the Church of All Worlds practices a less traditional view of marriage and sex. As you can imagine, the 1960s enjoyed this aspect of the novel quite a bit.
In defense of Mike's Church, Jubal argues that sexuality should never be used to hurt anyone; but Mike's sexual advances actually caused Ben quite a bit of distress and harm.
Women are generally more sexually aggressive than men in the novel.