When a certain emotion is not properly used or if it is abused, the corresponding chakra is disturbed and diseases related to that chakra manifest themselves in the body.
If you drink one cup of tree, intensely and physically living it, you will be liberated from tea! Same law holds for sex!
When body moves with the emotion and mind does not respond, one is liberated from the emotion. The biggest problem with sex is that the mind adds and deletes a lot for the emotion of sex.
There is you and your fantasy of your spouse. Then there is your spouse and your spouses fantasy of you. Not to mention multiple fantasies of the same person. Now, there are at least four persons in the same house. Naturally it is chaos.
Fantasy reduces sex to lust.
When you let go of fantasy, your spouse is divine to you. When a couple let go of their fantasies, they are shiva and parvathi (Gods in Hindu mythology).
Fantasy reduces god to nature.