Actually, I reject it because it's false.
What problems?
Lets' say you reject the supposition that marriage is the metaphysically comprehensive union between . . . and that its purpose is to attach . . . etc. What do supporters of ssm think that marriage is? Here is one popular answer:
Marriage is the lifelong union of two persons who love each other. Marriage functions as a way of publicly acknowledging ones love and commitment for their spouse. Because same-sex couples are capable of loving in the same way as everyone else, the law ought to make provisions for recognizing their unions as legal marriages.
Love alone cannot be sufficient for legal recognition. For one, not every loving relationship is afforded legal recognition. There are many different kinds of valuable social relationships that are simply not relevant to the public good in the way that marriage is. Companionships, for example, involve love, but nobody is calling for the government to legally recognize friendships. Love certainly
motivates a couple to enter into marriage, but it is mistaken to think that marriage is
essentially about love.
Key to a particular type of relationship being subject to legal recognition is its serving a
public purpose. Social institutions are afforded legal recognition in virtue of their serving some good to society-at-large, not because those involved may happen to love each other. There must be something
intrinsic to a certain kind of relationship that bears on the common good for there to be a legitimate state interest in regulating it. Now what public purpose would legally recognizing a loving relationship serve? Love is an essentially private matter that involves only those people in a relationship. The state has no business poking around in this domain of life, since the state exists to regulate public goods and institutions. Some bring up the various benefits and incentives already associated with marriage as a reason to legalize same-sex relationships. According to this argument, same-sex unions should be afforded legal recognition in order to take advantage of the benefits that opposite-sex couples currently enjoy. But this is clearly question-begging. Why should
anyone deserve these benefits to begin with? It cannot be because they love each other, since thats the very issue at stake.
Some libertarians have used this rationale to argue that the state should not be involved in marriage at all. If the position described in the preceding paragraphs captures what marriage amounts to, they have a point. But in fact many of us have the intuition that marriage is something special, something that the state should involve itself in. The question then becomes:
Why? Whats so special about marriage that could justify this? And the only reasonable answer to that question is because marriage has to do with the union of man and a woman and their children.
By denying this and accepting the supposition that marriage is just "recognizing loving commitments," then not only does this serve no public purpose, but it also commits you to recognizing any concievable configartions of individuals who are "lovingly committed to one another" are "married."
No, it's not a mere triviality. It calls attention to the fact that the system you propose is discriminatory even when you claim that you're imposing the same condition on everyone.
No it's not. On my view, everyone can marry someone of the opposite sex. There's nothing discriminatory about that. For on my view (indeed, the correct view), the public purpose of marriage is to attach mothers and fathers to their children to one another. And so, on this view, having two men or two women "marry" one another just makes no sense. It would be no more discriminatory to disallow two men or two women (or indeed 5 men and 10 women) to "marry" on this view than it would be "discriminatory" to disallow a man to become a member of a women's debate club, or to disallow a blind man from getting a drivers' license, etc. How many times do I have to explain this? Is this is just too hard to comprehend?