It’s quite simple, really, as to why sex is to be reserved to be only between husband and wife, between one man and one woman. It’s called holiness.
To understand this better let’s look at what G-d has reserved only for Him. The incense that G-d required to be burned at the temple was a very special incense, a formula only allowed for and reserved for burning unto Him in His temple. It wasn’t just any incense, it was holy, which means set apart, special. It contained eleven different ingredients, only four of which were written down in the bible. It was so holy, that it’s contents couldn’t be fully written down for all to see in Exodus 30:34. Many of the ingredients are extremely rare and can only be obtained in exotic and distant lands with respect to the location of the temple. Everything in the temple was created only for the sake of G-d’s honor, but the incense was super special both in its content and how well the knowledge of that content was guarded. It was never to be used for anything outside of honoring G-d in worship. Nothing else was to try to come into its place. We see this in Leviticus 10:1 with reference to the “strange fire” that Aaron’s sons attempted to use in the temple.
Our intimacy should be equally as well guarded and reserved, being holy and only for the purpose of honoring our spouse. Nothing should ever come into the picture that replaces or substitutes that holy intimacy with a “strange fire” because it will defile the holiness. And when I say intimacy, I’m not talking about just the physical intimacy. Our emotional, mental and spiritual intimacy should also be equally as guarded and reserved just for our spouse. Our spouse should be the most intimate person in our lives other than G-d Himself.
That’s how it was intended, from the beginning.