Marriage is not just about more intimacy. It is about merging with one's sexual other half or counterpart, a complementary sexual other. Erotic desire for what one is as a sexual being is sexual narcissism or sexual self-deception: an attempt at completing oneself sexually through merger with a sexual same. Most people intuit something developmentally deficient about being erotically attracted to the body parts and essential gender that one shares in common with another. See my online discussion in "Why the Disagreement...?" pp. 30-46 here; and my published entry on "Homosexuality" in New Dictionary of Christian Apologetics (Intervarsity Press), 327-32.