- Unitive: Sex should draw the husband and wife together into a more perfect union with each other; that through it, "the two become one flesh" (Genesis 2:24), in a way fully knowable only to God.
- Procreative: Sex should call forth new life, or at least be open to it. Authentic love isn't jealous, it's creative.
To understand sex, I think you need to understand the Trinity. In the Trinity, God the Father loves God the Son, the Son is Beloved, and the manifestation of that Love is the Holy Spirit. Fr. Barron does a good job explaining that here. It's precisely because of that Love that God made the Universe and everything and everyone in it. So the Trinity is both a Unity and Creative. So the very same love that draws the Father and Son into an unbreakable Divine unity is the love that causes them to create the universe.
Our love should be like that. When a husband loves his wife, the two really do become one flesh, and that physical and spiritual intimacy should desire to produce new life. This willingness, and even desire, to produce new life is a sign that this is something spiritually healthy, rather than simply hedonistic.
So in marital sex, the two become one (Gen. 2:24), but the two also become three, when their love takes the form of an unborn child. So sex is designed to be cause the two individuals to form into something better, something both three and one, a reflection of how the Trinity is both Three and One. So each child brought into the world in this way is stamped with an image of the Trinity simply in his conception.
It's why we can speak of the Trinity as the first Family: each Member of the Trinity pours Himself out completely for the other Two. Just read the Scriptures, and you'll see that instead of proclaiming their own greatness, each Person of the Trinity focuses on how great the other Two are:
- The Father praises the Son in Matthew 3:17 and Matthew 17:5, and is the One who testifies about Him (John 5:31-32);
- The Son does the will of the Father (John 5:19-20), while refusing to testify on His own behalf (John 5:31-32);
- The Father pours out the Holy Spirit upon the Son, while the Son then pours out the Spirit upon us (Acts 2:33).
- Perhaps the highest complement that Jesus can pay is paid to the Holy Spirit: "It is for your good that I am going away. Unless I go away, the Counselor will not come to you" (John 16:7);
- The Holy Spirit is sent by the Father, in the name of the Son, and teaches us about the Son, Jesus (John 15:26), and it's only through the Holy Spirit that we can say that Jesus is Lord (1 Corinthians 12:3);
- The Spirit offers the Sacrifice of the Son on Calvary to the Father as atonement for our sins (Hebrews 9:14);
- The Holy Spirit leads us to the Father (Galatians 4:6; Ephesians 2:18).
Just look at Jesus praising the Father while filled with joy through the Holy Spirit (Luke 10:21), and you have a perfect image of Love. The Father is sending forth His Holy Spirit, the Holy Spirit is filling Jesus with love, and Jesus is pouring out praise for His Father. Jesus immediately praises their Trinitarian Unity (Luke 10:22).
This is one of the reasons that contraception is wrong. Using contraception to attempt to stop the procreative aspects of love while taking the other aspects is a perversion. We're trying to control and contain love: allow the two to become one without allowing them to become three. It just doesn't work -- couples who contracept are at a much higher risk of divorce than couples who don't. Instead of pouring yourself out completely for your spouse, you're holding something back.
If a man sat down to a meal, chewed up his food, sucking out all the flavor, and then spit everything out, we'd view that an unhealthy and abnormal approach to food. It's not a coincidence that food tastes good: the very reason food is pleasurable to eat is because our bodies need it to live. The pleasures of eating are a subtle way we're lead into doing what we should be (eating). To extract the pleasure without satisfying the body's need for caloric intake would render the pleasures of food meaningless.
We'd recognize God's plan (make food enjoyable, so people eat, so they don't starve), and decide to actively thwart it. Likewise, the reason sex is pleasurable is because we're called to be fruitful and multiply (Gen. 1:22) and to be drawn into marital union -- the pleasures are a way of encouraging that. We can know this by the very design of sex, or by looking to the animal world. There, scientists tell us that animals don't enjoy sex, but do it simply out of a reproductive instinct. So sex is, by its very core, intended towards reproduction. That it's also unitive and pleasurable for most spouses simply points to additional graces God bestows upon mankind. To thwart the core reproductive reality of sex, to chew it up and spit it out, is an offense against both authentic sexuality and against God's design.
Instead, couples should model their love off of the Trinity. Pour yourself out for your spouse, draw into perfect union with him/her, and be open to the manifestation of that love in the form of children.