Argh! Now the debate is turning towards the end that I disagree with. Hopefully this won't incite more flaming replies (no pun intended; "flaming" in the computer sense of the term)...
The problem I have is when this topic turns into being about human rights... Marriage (esp. when you are talking about the legal definitions of it) is not a right; it is merely a contractual construct that dictates the interaction between people that form a family unit. It become a definition that can then be applied towards other laws, such as beneficiary laws, family law, tax laws, etc. As such, as long as the definition is fairly applied to all people, it shouldn't be considered to be infringing on a person's right to equal protection as defined in the 14th Amendment, even if the law appears biased, esp. if the person arguing for homosexual marriage based on a "human rights" issue uses a more common definition of marriage as being a union between two people in love. Furthermore, since "marriage" is defined in family law, the separate states are the ones that should be defining how they define marriage, and the federal government should be reflective of these many laws when it comes to a context in the national jurisdiction (eg how to handle retirement benefits of a federal worker with respect towards the person's marriage partner). So I agree that DOMA should have been struck as unconstitutional (since it would have been an unequal treatment of homosexual couples that were legally married in a state that recognizes same-sex unions as being legal marriages), and for the most part I agree with the Respect for Marriage Act.
The actual human rights of homosexuals are already applied in the law. Homosexuals are allowed to associate with, date, speak with, and love with whomever they want as protected under the 1st Amendment. And in part this argument was used when the courts correctly struck down sodomy laws in Lawrence vs. Texas (in addition to the Equal Protection clause in that many sodomy laws were only being used against homosexual type of sexual congress, but not when it came to similar actions in a heterosexual relationship) allowing people to have sex in any manner they wished to express themselves.
Thus if a state decides to define marriage as being between a man and a woman under some specious argument like wanting to increase that state's population, then I actually don't see a problem with it, as long as a) that law is equally applied (eg allowing for the "loophole" of having a homosexual man to marry a homosexual woman) and b) it doesn't infringe on other rights (eg the law doesn't ban an actual homosexual couple from cohabitating, since I feel that this does infringe on a person's 1st Amendment rights to freely associate). But by the same token, most reasons for banning same-sex marriages seem to be based solely from a religious context, and because of that I can't really understand why a state would decide to make such a law if that state is also trying to keep that separation from the church.