The best argument so far is that gay marriage is a political wedge that will always energize the Right against the Left. It seems if you want to lose elections, just talk about gay marriage.
Other arguments have not been very compelling:
- Voters have consistently voted against it - ("against the will of the people") I have little doubt that if there was a vote to keep slavery legal in the South, that wouldn't make it something you should advocate. Part of the problem of democracy has always been preventing the majority from bullying the minority.
- Most religious groups opposite it - same problem as above group; many religious groups also favor it. Even if the majority is in favor of something, that doesn't make it right or wrong.
- Courts should not decide this issue, the people should (source: Brian S. Brown, executive director of the National Organization for Marriage) - counter example: Brown vs. Board of Education; courts decided against discrimination, few on the left thought this was a bad idea then or now.
- Marriage is for children - this may have been true 100 years ago but clearly isn't accurate now.
- Bad for families and children - research on this isn't compelling.
- Its unnatural - sounds too much like the argument against interracial marriage.
- Slippery slope - people will marry animals or inanimate objects. This is easily addressed since there's a fallacy named for this.
- Fundamental change to social institutions should happen slowly - the left loves equality and feels changes since the 50s have been for the better.
- Gay people are fundamentally unequal - This is probably the weakest argument given some of the greatest thinkers in history. A few examples include Da Vinci, Wittgenstein, Francis Bacon, the greatest American poet (Whitman) and a math genius who probably had a lot to do with winning World War II for the Allies (Alan Turing).
- What is really advocated? This is a tough one. Lefty groups should be clear why they do or do not support polygamy, which is not an unthinkable next step. The focus now is on equality in the eyes of the state, but those advocating for gay marriage are tacitly suggesting a definition of marriage as only between two people while they dismiss a definition that excludes same-sex marriage. Two women who love the same man may wish for the same equality. Are there polygamous Leonardo Da Vinci's? Is one necessary to give group its rights?
- The Left is advocating marriage by disparaging it. One of the frequent tactics is to point out that if famous morons can get married and divorced at the drop of a hat, Marriage isn't so sacred after all. This makes for good late-night monologues but is ultimately self-defeating. Its like arguing that voting is meaningless to avoid giving advocating women's suffrage.
- Anyone who doesn't want gay marriage is a bigot. If your beliefs do include marriage only as a union between a man and a woman, that does not make you a Nazi. Bigotry is a "person obstinately or intolerantly devoted to his or her own opinions and prejudices" (source), which is something anyone can be if they don't consider contrasting positions.
- Hospital policies against non-family members during possible end-of-life moments. This is a problem with legal issues / hospital policies. You should be able to decide you want or don't want at your side when you're dying. Marriage is connected to this but not the only solution.
- There is enough war and death in the world without telling people they can't love each other. A meaningless emotional argument, it is similar to the hospital argument in that its both true and obvious, but marriage is not and was never a permission to love. On the other hand, taken together they represent a legal challenge to a long list of other anti-gay laws that may represent a challenge to one's right to love someone of the same sex. If the state allows union, it will be forced to contend with many other issues of equality and equal protection.