The current argument in favour of homosexual marriage is that, without it, the institution of marriage is discriminatory, just as laws against miscegenation were 50 years ago. It is a violation of civil rights to keep homosexuals from enjoying the same benefits as heterosexual couples.
One can make this case, but it has two serious problems, one ontological and one practical.
First, the definition of marriage is and always has been a union of a man and a woman. Some counter that marriage has previously been defined as the marriage of a man and woman of the same race, and since that definition was overturned, why not this one? But marriage has only sometimes included laws restricting the race, religion, or class of the married couple; it has always and everywhere meant a union of a man and a woman.
(In the case of polygamy, it is a man and a woman, and that man and another woman, etc.; in each case, one man and one woman celebrate a marriage ceremony. One man's wives are in a single family, but they are not married to each other.)
Is there a difference between restricting which men and which women may marry each other, and restricting whether men may marry other men? It seems an obvious answer to me. You can argue that the difference is not sufficient to justify excluding same-sex marriage, but I think you are not being honest with yourself if you say there is no difference at all. Creating a form of "marriage" that has never been practiced in history is changing the definition of marriage -- which may be justified, but it is not fair to call it discrimination.
Of course, it is discrimination of a sort. Every law -- every definition -- "discriminates" in some way; otherwise, there would be just one meaningless law that encompasses everything. We don't allow ten-year-old children to get married, for example. This may seem a trivial example to you, but consider how trivial the idea of same-sex marriage seemed to most people until very recently; and some people (among them Hillary Clinton) have argued for granting legal status to minors, at least in some cases. And it is not only a theoretical case, because different states have different rules about the minimum marital age. Why should a 15-year-old be allowed to marry in one state but not another? And why should he require, not only parental consent, but judicial consent? If we want the government to be completely non-discriminatory in these matters, I don't see how anyone could tolerate a law that gives it veto power over the marriage of some people and not others.
Marriage laws also discriminate in only allowing each person to marry one person at a time. If same-sex marriage is allowed, I don't see what legal or moral basis will be left to oppose polygamy. This issue is not far from the surface, because there are thousands of people practicing polygamy illegally in the United States right now. At least there is no law against private same-sex marriage in the United States, while polygamy is a misdemeanour or even a felony in many U.S. states. Other Western countries, including Britain and Australia, recognize polygamous marriages for the purpose of distributing welfare if the marriages were performed in other countries; inside of Britain and Australia, wedding a second person is illegal. Unlike same-sex marriage, which has no history, polygamy is permitted or tolerated in over a quarter of the world's countries, so the argument in favour of it seems prima facie to be much stronger than same-sex marriage.
The practical issue with labelling heterosexual marriage "discriminatory" is that, unlike miscegenation laws, marriage was never set up to discriminate. It was instituted as a way of regulating sex and families, and only ignored homosexuals because they were irrelevant to procreation. Therefore, its history is completely unlike the history of race-based marriage laws.
Heterosexual marriage is also different than miscegenation laws because it is mandated by the major religious of the United States. Certainly, some Christian denominations have changed their views of homosexuality, but there can be no question that it is prohibited in both the Old Testament and the New Testament (Corinthians I 6:9). It is unlikely that all Christians, or even a majority of them, will completely abandon their views.
To this, those in favour of same-sex marriage have two responses: (1) why should we care what the church thinks?, and (2) it's a matter between two people who love each other, it's no one else's business. But a private marriage between two individuals is no one else's business; requesting legal recognition of a marriage is definitely other people's business. Still, why should anyone really care if the government recognizes same-sex marriages? It doesn't require them to change their practices, so they are just being obnoxious in opposing other people's happiness.
One reason for opposing same-sex marriage is that it challenges the whole definition of marriage. That's why I think a civil union is reasonable compromise: it can give the legal advantages of marriage without offending a whole group of people who think it is wrong. As far as people being obnoxious in opposing others' happiness, I think those who insist on same-sex "marriage" instead of civil unions are the guilty parties here. (Yes, I am aware that the Connecticut Supreme Court ruled that civil unions are discriminatory. I think it is very wrong, but I don't want to get into the matter here beyond saying that I disagree with their assessment.)
The other problem for Christians allowing same-sex marriage is they might very well be forced to recognize those marriages contrary to their principles. There has already been a case where a baker was investigated for not making a wedding cake for a same-sex marriage, and that was in a state that doesn't even recognize same-sex marriages! The other notable case concerns Catholic Charities, which was forced to end its adoption services because it would not adopt to same-sex couples. It is true that Catholic Charities received some state funding for its services, but it is not as though it could drop the state funding and continue, because adoption is strictly regulated by the government: they would lose their license if they did not follow the anti-discrimination law.
By formulating traditional marriage -- i.e., marriage as it has been always and everywhere -- as a form of discrimination, same-sex marriage advocates have made this issue much bigger than it needed to be. If they win, no one will be able to practice their religious beliefs regarding marriage. Businesses will have to often benefits to same-sex couples, and anyone else associated with marriage -- such as the Oregon baker -- will have to sacrifice his religious beliefs or his profession, because I don't see what accommodation will be possible if same-sex marriages are ruled a civil right.
Should we care? Don't we have a separation of church and state? Yes, but we also have freedom of religion. Obviously, none of the framers ever conceived that freedom of religion would come in conflict with civil rights on this point. This is another area where same-sex marriage is drastically different than miscegenation laws. There is nothing in the Old or New Testaments in favour of discrimination against people of different races; historically, churches have been at the forefront of defending the rights of Native Americans and African Americans against slavery and oppression. This is new ground.
I don't know what the liberal response to this situation is, but I suspect it would be, so what? We have separation of church and state; you can only practice your religion as long as it doesn't conflict with government rules, including basic fairness. In fact, I would agree with this to a point: we only tolerate religion within certain bounds. I don't expect this Sikh man to win his lawsuit against gun control on religious grounds. Where I disagree is on the definition of basic fairness. Freedom of religion was established to allow people to hold different views of fundamental issues. By brushing aside religious concerns about same-sex marriage, people are interfering with religion and religious practice in a very fundamental way. Their idea is that what seems perfectly obvious to them -- that heterosexual marriage is discriminatory, pure and simple -- should be the law of the land for everyone. This is very curious for a political philosophy that promotes diversity as one of the highest goods. They avoid the issue now by claming that same-sex marriage has nothing to do with anyone else, but this will quickly be shown to be false if the Supreme Court rules it a civil right. And then we will hear no defense of diversity in views on marriage. Diverse opinions are great, they think -- as long as they don't touch on anything we disagree with.