The single greatest argument I've ever heard made against extra-marital sex isn't about sex at all; it's about a bagel. In fact, I very much doubt the author of the argument ever intended it to be used as I am about to.
It comes from a book I read a few summers back. I don't recall the title, but I do remember it was one of those great, emotional Protestant reads about falling in love with God, and that the chapter which the following account was drawn from dealt with acheiving intimacy with Jesus.
The story goes as follows: a girl, perhaps the author herself, was reading Scripture and meditating on the Institution narritive, or Luke 22:19 I believe, and she found herself being drawn deeper and deeper into prayer. She could feel Jesus tugging at her heart, and it was awesome. She got the distinct impression that the Lord was calling her into communion with Him, that He desired to be with her in a new and more intimate way.
"Communion Lord?" she wondered, thanking Him for such a beautiful image and continuing in her prayer. But the urge became stronger, more insistant, and she realized that God wasn't calling her to "communion" but rather to "Communion;" the big- C kind.
So she followed her feelings, and acting on the natural desire, the human longing for union with God, she made her way to the nearest convenience store and picked up a bagel and a bottle of grape juice. God was inviting her, she believed, to partake in Communion with Him, participating in the redemptive offering of His body and blood.
And He was inviting her. The longing she experienced as she read the words we hear repeated in the Eucharistic prayers, "Take this all of you and eat it, this is my body, given up for you..." Those beautiful words, intoned by the Logos, the Word Himself, to His followers in the upper room, those words were designed to inspire longing in the human heart.
Similarly, our sexual desires are designed to draw us into deepening intimacy with our spouse and, ultimately, the Trinity. The urge to give and receive love in this most personal expression of human love is God-given. But there's a catch: just as He created and endowed us with these desires, so too did He create the context, the end toward which our desires are ordered to. And while we can recognize the goodness in our friend's desire for intimacy with her Lord, we also plainly recognize that her experience of "communion", however good it felt, was not a true participation in the Eucharistic banquet He has prepared for us.
Her experience was with simple carbohydrates. A bagel and grape juice. Symbolic, but ultimately falling short of the sacramental reality God gives us in the Eucharist. Similarly, sex outside of marriage, however right and good and pleasurable it might appear, is no more a sacramental participation in the divine love for which we were designed that that bagel was the body of Christ.
Simply put, extra-marital sex isn't sex at all, but a shallow mimicry of the fullness of intimacy expressed and experienced in the spousal embrace. You might be in love, it might feel right, but it's not sex, not as He designed it. And though it looks and feels like the real deal, the fruits of such sexual union are exactly the opposite of His intention for this life-giving communion of persons. Instead of unity there is division, instead of trust, suspicion, and instead of joyful self-gift, there is selfish lust and insistance of one's own way.
And that bagel might taste good, might help you experience the historical reality of the last supper, might put you in the right frame of mind to meditate on the Gospel if you're a hands-on learner, but it's not going to increase your sanctity; it's not going to save you, it hasn't the metaphysical power to do so. And sex outside the beautiful boundaries of the sacrament of matrimony isn't really sex at all. Where He would give life we invite death, and no matter how right it feels, our subjective experiences of He who is Objective Reality don't change a thing.