“Do you not know that your bodies are temples of the Holy Spirit.”
The Bible teaches that we do not live in a world absent God. But more than that, God is not merely one more piece of “furniture” in the universe. He’s not something that can be ‘added to’ or ‘removed from’ the world without consequence. Rather, if God exists (and He does), then His Being undergirds all reality. And humans, being made in his image, are sacred in their being (both soul & body).
This is why humans are charged not to murder, for man is made in the image of God, and is sacred — including his body. The Human Body, then, has a value that is unique in creation. This is not to say that all creation is invaluable, but rather that humans–and their bodily integrity–have a certain additional value in virtue of their unique status as God’s image bearers.
When we lust, we are taking, we are using, the body of another human being for ourselves. We are profaning something sacred. We are taking something holy and treating it as something common, as if it were there merely to satisfy another appetite — but it’s not. It’s involved in the very creation of more humans. You cannot separate the meaning of the human body from the object of the human body. If you could, one could be violent with human bodies — destroying them — just as you can do with an inanimate object, such as a tree or rock. But we know that’s wrong. We know it’s wrong to be violent with other humans, because they are not mere objects.
When we lust after other humans, seeking to take them for ourselves, we are denying their meaning and value. Even if the other person we’re lusting after is a willing participant, that does not remedy the issue. It only means that they themselves are denying what their body is for, that it is something sacred. And when we affirm them in that denial, that does not absolve us, but rather means we are encouraging them to deny themselves. Imagine if someone were harming themselves — say cutting — and they knew other people would pay them money to cut themselves, and they took a certain pleasure in other people viewing them cut themselves, would the person who pays the cutter in order to watch them cut be absolved? Because the cutter is willing? God no.
And when we engage in lust, pornography, fornication, whatever it may be, using the body of another, and affirming them in that, we are profaning the sacredness of their body.
God made humans sexual creatures. Sex isn’t bad or evil. The union between a man and woman in intercourse is deep and profound. It isn’t just material. The Bible says, “the two shall be one”. But that oneness is meant to illustrate a covenant relationship. It’s an outward expression and demonstration to the other that you are saying, I am entirely yours, and that each is saying that to other. To do that with your body, but not your being, devalues yourself and them.