All true religions in their pure, original forms considered sex to be sacred. Why? Because our true nature - which is pure and without fault - includes sexuality. In other words, God created sex. Mankind corrupted it. Therefore, by returning to pure sexuality, we can also return to a state of divinity.
We were driven out of Eden through the doors of sex, thus we can return to Eden only through the doors of sex. - Samael Aun Weor
Browse the scriptures of all the world traditions to see the universal knowledge (gnosis / jnana / daath) of sacred sexuality.