Here's the difference, whether you're religious or not: a union between a man and a woman is designed for the bearing and raising of children, whether or not that actually happens in every case. Moreover, almost every union has the potential for producing children. Plenty of women who thought they were post-menopausal have suddenly found themselves a happy mother made - and the early 50s, even the mid-50's, ain't out of the woods yet, believe me! (I'm 54). And I personally know two couples who tried everything to get pregnant (including in vitro), were told they were unable to conceive, adopted a child, and then suddenly turned up pregnant.
Even if you don't believe in the religious aspect, marriage between a man and a woman is socially, potentially, and ontologically different from a necessarily barren homosexual relationship.
P.S. . . . annulments are not that difficult (or expensive) to obtain, depending on circumstances.
We could use the same description to define something way more simple: a heterosexual sexual act. That was nature's design, anyway, at least if we look at the animal world, where no special ceremony is necessary. Human societies took that act and confined it to one man, one woman, at least for the common people. The king always got to have multiple wives and concubines. Even Biblical figures did this. But I'm sure you're aware of that.
Moreover, almost every union has the potential for producing children.
You can forget my potential marriage with my lady. See my comments above as to why it "ain't gonna happen". Given that, would you deny us the benefits that marriage would supply to us, even though we have zero possibility of producing children to protect with the societal institution of marriage?
Even if you don't believe in the religious aspect, marriage between a man and a woman is socially, potentially, and ontologically different from a necessarily barren homosexual relationship.
You mentioned adoption earlier in your response, homosexual couples can adopt children in the vast majority of states. They certainly earned their stripes in this endeavor when they took on AIDS babies that nobody else wanted. And with a pair of lesbians, all they need is a donor, and both of them can have children if that is what is desired. There's not a single thing that our society can do to stop them, if there were, we'd use that to prevent welfare mothers.
I know that socially speaking, it has been one man, one woman in most Western societies for a very long period of time. I also know that a mere two hundred or so years ago, the men that I deeply admire as the Founding Fathers regarded full political freedom as something that could only be extended to white, property-owning males over the age of twenty-one. Clearly, we've seen a social evolution from beyond that point.
Perhaps the institution of civil marriage, which is the only religious institution co-opted by our legal system, would evolve as well. That's what happens when things that should have stayed within religion become part of the law.