To get the first point out of the way: a taxpayer funded organization promoting homosexuality in Milwaukee estimates that between twenty and forty percent of gay men in Milwaukee are HIV positive. And this from people arguing for the acceptance of the homosexual lifestyle. Homosexuals really do "get AIDS and die", and at much greater rates than their heterosexual counterparts. Thus, while it is a concern for both heterosexuals and homosexuals, it is a much more pressing concern for homosexuals.
Through marriage, I can extend to another the same gift of life given to me. Two homosexuals may be fond of each other, but their relationship will never give life to another person. The ability of male and female to reproduce, and that marriage is oriented toward reproduction, distinguish it from other loving relationships.
There are heterosexuals who, like homosexuals, use one another purely for personal pleasure. Their relationship is contingent on receiving, rather than giving. But these heterosexuals aren't asking us to consider them married; on the contrary, they often go to great lengths to assure people of the just the opposite.
Were it not for the desires of the flesh, so to speak, the relationships between homosexuals would be merely very good friendships. There's nothing wrong with loving another person. On the other hand, homosexuals take the human capacity for reproduction, and instead of using it to give life and love to others, use it for their own personal pleasure. Again, there are heterosexuals who do the same thing - but they aren't asking us to consider themselves married.
I can understand and respect the love two people have for each other. But I also recognize that to use another person as an object of sexual pleasure is to deny them the dignity due someone made in the image and likeness of God. In short, homosexuals are worth more than their partners esteem them, and are content to continue a self-deprecating relationship. They either do not understand their dignity and worth as human beings, or don't care. I can also recognize that many of them are probably very confused with respect to what a good relationship should feel like, and are intimately aware that their sex lives make them feel undignified, but don't quite understand what to do about it.
Would it be enough to regard two homosexuals as one regards the cohabitating couple? Would that be granting them enough dignity? Because I can recognize the value of loving another person independently of their sexuality. But I can also recognize the value of giving to others the gift of life, in a selfless act of sacrifice, and esteem it more highly than two people who are merely happy to be together.
Recognizing this difference is not unjust discrimination; rather, it is simply being truthful about reality. Unjust discrimination stems from pretending a an insignificant difference is significant. This is what the proponents of gay marriage are trying to argue: that the self-giving sacrifice of marriage, that the complementarity of the sexes, that the action of God in bringing two people together, is not significant. This view is particularly insulting to those of us who are married and do make significant sacrifices for marriage, which extend far beyond what a gay couple is willing to give. By the very nature of their homosexuality, gays have intentionally ordered their relationship in such a manner that they will avoid giving to others the same gift of life they received from their parents. They will avoid the struggles of raising children and facing hardships together, yet wish to be esteemed as those who do.
Now, if you don't believe in God, don't believe humans to be made in the image of God, and believe that marriage is nothing special, nothing permanent, inconsequential in the raising of children, then why would the state recognize it at all? If you want the state to affirm that a loving, committed relationship is a societal good, then wouldn't a loving, committed relationship arranged for the purpose of bringing new life, and more love, into the world be even better? If you recognize marriage as an act of God, you automatically exclude homosexuals from it; if you recognize it on its earthly merits, it is still different, and better, than the so called "gay marriage". If you wanted to recognize the love between two people regardless of their sexes, you can do so, but to call it marriage is being deliberately dishonest.