Heather Mac Donald offers an anti-marriage equality argument you don't see very often; gay marriage might further depress marriage among African Americans. She writes:
It is no secret that resistance to homosexuality is highest among the black population (though probably other ethnic minorities are close contenders). I fear that it will be harder than usual to persuade black men of the obligation to marry the mother of their children if the inevitable media saturation coverage associates marriage with homosexuals. Is the availability of homosexual marriage a valid reason to shun the institution? No, but that doesn’t make the reaction any less likely.
What are the chances that gay marriage would further doom marriage among blacks? I don’t know. Again, if someone can persuade me that the chances are zero, then I would be much more sanguine. But anything more than zero, I am reluctant to risk.
This is a close equivalent to the argument about gays in the military: since some feel they have cooties, it would wreck the whole enterprise. At least Heather concedes the unfairness of it - using bigotry as a reason to perpetuate bigotry - but it does fall into the Byron York trap of treating one minority as somehow not being real or equally valid citizens. And while this might have remained a cruel, unjust but utilitarian model in the past (however trivial homophobia might be as a factor in shoring up African-American marriage), it doesn't really work in a world where gay couples and their kids are public, unavoidable realities.
Once that happens, the dynamic is a little different. It becomes actively denying basic rights to one group in order to placate the prejudices of another. I can't see how this could work in an open and free society. It's so baldly discriminatory and unfair. One of her readers tries a different tack:
Virtually no law that we’ve made in human history can be guaranteed against negative unintended consequences. Ought we make no laws, then? Of course not. We try instead to make laws on the basis of reasonable, considered benefits to the law against reasonable expectations of what we are trading off.
Or perhaps it’s that you have a different framework for considering the role of the government and individuals. You seem to speak of marriage as if it’s a benefit that the government discretionarily hands out to individuals. Therefore, before we extend that benefit to more citizens, namely gay citizens, we ought require a compelling case to be made that such citizens are worthy of the right, including proper consideration of the potential negative consequences a detailed plan a guarantee in fact that these consequences can be avoided.
If that’s your default operating framework for government, it seems to me decidedly not conservative. For I believe the conservative position starts from the basis that individuals have a natural right to freedoms in their affairs, and that the essential role of government is to protect these rights, imposing limits upon them only when such rights can be shown to interfere with the rights of others or the collective good.
In the case of marriage, then, I think your assumed default position is exactly counter to a conservative stance. The onus should be those who seek to restrict an individual’s right to marriage to demonstrate why such a right interferes with the liberty or well being of the collective as a case against it. Failing such a compelling argument, the government ought take no position. Here you seem to be arguing for the opposite: an activist role for the government in regulating marriage, with your fundamental point being that we ought subject the freeing of some of the regulation (the prohibition against same sex partners) to higher scrutiny prior to agreeing to it.