I don’t think I could have said it better myself. A Christian’s very basis of understanding humanity and sexuality is rooted in God. This goes back to the formation of our rebel band of lovers 2,000 years ago.
The story Christians have been telling for 2,000 years goes something like this: The God who made the Universe is also, by his very nature, Love, and he made human beings with a very lofty vocation. Humans are meant to reflect His glory in the world; to be like God, that is to say, to be lovers and creators. Everything in the Universe has been put here to be used by God's children to reflect his loving glory — and to teach them about God's love. This is particularly true, or so the story goes, of the unique sexual complementarity between men and women. The sexual act is meant to reflect God's love by fostering a union at once bodily and spiritual — and creates new life. The complementarity of the persons in a marriage reflects the complementarity of the Persons of the Trinity, and the bliss of marital union is an inkling of the bliss of the union of the Persons of the Trinity. The fruitfulness of the marriage act reflects that God is a creator and has charged man to be an agent of his ongoing work of creation. And, finally, if God's love means total self-giving unto death on a Cross, then man and wife must give themselves to each other totally — no pettiness, no adultery, no polygamy, no divorce, and no nonmarital sexual acts. According to the story that Christianity has been telling for 2,000 years, Christianity's view of sexuality isn't some encrusted holdover from a socially conditioned patriarchal era on its way out, but is instead deeply connected to its understanding of who God is and what human beings exist for.