There are no reasonable arguments against it. But, just for kicks, let's review those arguments that are often put forth by those who oppose equality for gays and lesbians, and I'll rebut each one. (Note, some of this content originates from comments I made on a post at Eric Ethington's blog.)
Homosexuality is a SIN
Due to the religious freedom we enjoy in this country, you have the right to believe that homosexuality is a sin. Feel free to believe that, abhorrent though your belief may be to me. But fortunately, due to those same religious freedoms, the laws of our nation cannot show deference to any one religious viewpoint. This is why we do not force rape victims to marry their rapists (DEU 22:28-29). This is why it is permissible for women to go to the store without the accompaniment of their husband. This is why you can enjoy your cotton-poly blends and delicious lobster bisque. This is why an expression of love and commitment should not be barred simply because your particular brand of religion frowns upon it. Despite what some of the pro-Prop 8 propaganda said, no one is going to force any church to perform gay marriages. But the government should not issue marriage licenses based upon a religious standard.
Marriage is about having children, and gay people can't reproduce
First off, gay people can reproduce. And I'm not talking about through teh evil recruitment ploy the right wing talks so much about. Yes, gay couples cannot produce actual biological hybrids of themselves, but they may choose to have the biological child of either partner.
But this is all irrelevant anyway. If marriage were about having children, then we would require fertility tests and pledges to reproduce upon signing marriage license. This, of course is ludicrous. There are countless heterosexual couples who -- by choice or by fate -- do not produce young. Furthermore, many couples, gay and straight alike, choose to adopt.
But gays will make their adopted kids gay too!
Finally, we're getting away from religious objections and to actual testable claims. I'm going to start referencing the American Psychological Association's sumamry and review of the science done so far on lesbian and gay parenting entitled (appropriately) Lesbian and Gay Parenting.
Let's assume just for the moment that it would be a problem if this claim were true. This is not a position I hold, but let's put on our fundamentalist hat for a moment. Is the statement true? Are children of gay parents more likely to be gay?
The one word answer? No. (see here, the section entitled "Sexual Orientation.")
But children of gay parents will have to endure the pain of their parents being different
Do the children of gay parents face challenges? Sure. Kids are cruel. They'll latch on to anything that sets someone apart from everyone else and use it as a hook to make fun of a classmate. But this argument works just as well as a pro-segregation argument, or a reason to deny any group their equal rights. Furthermore, it's absurd. The failing in this scenario is not with gay parents or their children, but with anyone who would attack them either verbally or physically.
I can't really improve upon the APA report's take on the effect on children's social lives, so I won't even try. Just go here (same link as above) and read the section entitled "Social Relationships."
You can't redefine marriage!
Sure you can. It's been done time and time again throughout human history. Most every culture has some form of marriage. But there is massive variation in what "marriage" means. From one-man-one-woman, to one-woman-two-brothers to same-sex marriage (as in much of Europe). To say that marriage has always been "one man, one woman" is simply false. That isn't even true in the Bible.
But gay marriage will destroy traditional marriage!
This is probably the most common argument I hear. And yet it is the one that makes the least sense of all. Do you think Tim and Beverly LaHaye will get divorced once their gay neighbors down the street celebrate their vows with teh butt-sex? I'm going to bet no. Gay marriage is legal in Canada, many European countries and elsewhere. Yet heterosexuals still insist on getting married just as much as before. Funny how that works.
The main difference is that now, with the social stigma on homosexuality gone (or at least greatly reduced), there is less incentive for people to stay in the closet, meaning fewer closeted gays and lesbians entering into doomed hetero marriages. I have no idea what, if any, effect this would have on the divorce rate, but it certainly isn't the death knell of "traditional marriage."
If you allow man/man marriage, soon you'll have to allow man/goat marriage
Ah, the Rick Santorum argument. Unfortunately, the slippery-slope fallacy is, well, a fallacy. There is no legitimate parallel to be made here between same-sex marriage between two well-informed, consenting adults. Try again guys.
As an aside, the only instance of legal man/goat marriage I'm aware of is in Sudan, where the punishment for homosexual sex is death.
If you allow gay marriage, soon you'll have to allow polygamy
To be honest, this is the only argument discussed herein that carries any water whatsoever. And it's an issue on which I personally find it hard to make up my mind. Speaking for myself here, i don't see any rational reason why if two people wish to commit to each other, three or more shouldn't be able to do so as well.
The issue I have with polygamy is that, in practice, it's nearly always an unequal and controlling relationship. Your classic case is early Mormons and modern Mormon fundamentalists 9and, you know, the Bible), where the man more or less, "owns" his wives. In many cases in these communities, women and girls are forced into such marriages against their will. And that's not okay. My opposition to polygamy in these instances has a lot more to do with preventing exploitation.
For what it's worth, I do support right of same-sex marriage right alongside a lovibng,equal partnership among a polyamorous triad, for instance. But let's be honest. In general, when there is social and economic equality between men and women, we humans tend to pair off. There are not hordes of polyamourous people aching to register with the government. Even if we did allow polygamous marriages, it still wouldn't be the death knell of one-man-one-woman marriage. That would continue to be the pattern for the majority of Americans.
Has anyone heard any other reasons given by same-sex-marriage opponents?