You might want to review
this essay on the matter from Orson Scott Card.
Personally I see same sex marriage as objectionable mainly from a civil and societal standpoint. After all, there has been marriage since the dawn of civilization, and in all those thousands of years, there has never been same sex marriage.
Why not? Because every civilized society saw marriage as beneficial to civilization: Marriage resulted in families, and families ensure the next generations of citizens, taxpayers, soldiers, and everything else.
Many societies accepted homosexual behavior. They never accepted it as marriage. Because it isn’t marital, and can never be. It takes two opposite sexes even for marital relations occur. I’m not talking children; it’s more basic than that. Marital relations are impossible to homosexual couples, and therefore so is marriage.
Then if one really wants to get into the civilizational aspects of what marriage–real marriage–means to society, read Carle Zimmerman’s “Family and Civilization,” which surveys thousands of years of civilizations, societies, and how families were structured. Families evolve, but not wildly. They tend to transition from a patriarchal structure, to a domestic family structure. Those are both stable. When family structure seriously disintegrates, though, so does the underlying civilization. We are about there now.
So let’s go ahead and destroy family structure through contraception, divorce, fornication, and gay marriage. The next generation will have to live through the coming dark age of civilizational collapse brought on by the collapse of family structure.
The Catholic Church will, as always, help to rebuild.