Marriage used in by God as the best human analogy of His relationship to us. Think of the perfect marriage. One in all ways is naked and open and trusting and loving and honoring. Marriage is the used throughout Scripture to be the best human analogy we have of God's relationship with us. Sexand the ecstacy of sexual release is an analogy of how intimate and exhilerating life with God is and will be as we worship Him forever.
Sexuality is ultimately about intimacy.
Therefore, this leads me to my third point, that sexuality is ultimately about intimacy. It is the very culmination of an incredible, intimate relationship. I have a friend who used to talk to me quite often about sex when I was single, as he was already married. He used to tell me two things that are riveted in my brain. It's not about an act, it's about a whole day, week, lifetime of trust and intimacy.