Nature. We didn't determine it. It has determined us. It is not a matter of religious adherence, only regard for what is undeniably true, and common sense.
No argument surpasses the one the body has already made: Same sex bodies simply don't fit together, nor can their sexual processes accomplish what they are undeniably designed to accomplish. To call this "marriage" is as misguided as is forcing two, non-complementary puzzle pieces together and calling it a "fit," or calling one's eating and inducing vomiting "nutrition." By definition, each of these are unnatural.
Feelings and inclinations are not the basis of right, much less law, much less special privilege. Which of us do not have feelings or inclinations meriting our self-control? If we regard feelings and inclinations as basis for what is right, or law, what's to prevent any other variation? What's to prevent poly-amory (multiple person marriage), or marriage between, say, a father and a son? Nothing.
Yet this is what the U.S. Supreme Court declared today in legalizing same-sex "marriage." It is another raw act of judicial power not different from Roe v. Wade that, in effect, legalized abortion through all nine months of pregnancy. In so doing, they essentially validated feelings and inclinations as the new law of the land. To each his or her (add 49+ genders) own. They disregarded precedent, common sense and undeniable nature. The practical consequence is that morality is something each of us make up. All who might disagree are now discriminatory, hateful bigots.
Of course, in the grand scheme of things, all this amounts to our human game of playing pretend. God in Three Persons is the Supreme Court. Their objective Truth stands, Timeless and Unshaken, on solid Rock before all ages, a Law Giver and Law we did not create, but Whom has created us... which we can not so much break, only be broken by. The Church is the Steward, unwavering in articulation of this highway to heaven, it's contour and defining berms, sociologically validated as the true good for every person.
And, for those who smash into these, She will continue to be there to pick up the pieces.
Praise be God, the Father, Son and Holy Spirit! Your Kingdom will come, and Your will be done, here on earth as it is in heaven!