Gay marriage isn't real marriage because gay sex isn't real sex. The lack of complementarity bespells the lack of reality. Just because we cover up that reality with skewed labeling does not make it so.
We lost the argument on gay marriage when we allowed no-fault divorce. As Chesteron famously stated, "Frivolous divorce will lead to frivolous marriages." Such a thing has undermined the very meaning of marriage and the very meaning of sex, which says with the language of the body, "I am wholly yours and you are wholly mine." There is no such thing as saying, "I am wholly yours... for about 15 minutes." That abuse ruins the very nature of marriage.
Divorce came first, then contraception, then abortion. We attack the union of marriage, then its pro-creative capacity. Soon, for heterosexuals marriage was nothing other than the union of two people in love and that's it. This redefinition of marriage makes the whole gay marriage debate possible, and its outcome assured. After all, if marriage is really just the union of two people who love each other, who cares if they are the same sex? Only bigots would, as the public debate is framed.
There will be gay marriage legalization in the US because its basic definition has been prepared by the disasterous abandonment of marital love and truth by American Protestant Churches, followed by American Catholic individuals. While the Catholic Church has always resisted these deformations, Catholics in the pew have not, due to ignorance or outright dissent.
We catechists are to blame. The clergy are to blame. But once we are done blaming one another, can we get back to the tough task of fulfilling our mission as evangelists? Marriages depend on it. Families depend on it. The unborn depend on it.