1. It diminishes the meaning of marriage and therefore is an attack on my marriage:
Really? Does your neighbor's divorce diminish your marriage? Does your co-worker's infidelity diminish your marriage? If an other's divorce or infidelity negatively impact your marriage are you for a constitutional ban on divorce? Or criminalizing adultery? My guess is you are not and so your stance against gay marriage is not only hypocritical but misplaced. By including same-sex couples into the realm the marriage you add proponents for policies and issues that defend and support marriage, be they tax policies or public education for your children. Ultimately the more people who marry the stronger the institution of marriage becomes.
2. I don't want my children to go to school with the children of gay couples because it will teach them that there's nothing wrong with being gay:
This is one of the last forms of acceptable discrimination in our country. I'm not saying that other forms of discrimination do not exist but most attacks on other groups (based on race or religion) are met with strong opposition in the public. This line of argument points to a problem in your own life and your discomfort with gays and lesbians. Most of the data show that younger generations are more comfortable with gays than their parents and grandparents, so chances are your children will encounter homosexuality during their time in school and be OK with it. The easiest response to this argument is that there was a time not too long ago when the same arguments were made against integrating schools. Complaints from (mostly) white parents who did not want their children in school with "Negroes" was the driving force behind maintaining segragation as long as it lasted. Luckily we have, for the most part, agreed that this reasoning was bigoted and wrong and hopefully we will view the issues of homosexuality through a similar lens in the years to come.
3. My religion/church/spaghetti monster tells me that a marriage should be between one man and one woman:
For starters, let's remember that most major religions were founded over a thousand years ago and that they also contained passages like the following: "Six days may work be done; but in the seventh is the sabbath of rest, holy to the LORD: whosoever doeth any work in the sabbath day, he shall surely be put to death." So the next time you want to use the bible or some other holy book as a defense for your discriminatory policies towards gays remember that there's a lot of rules and decrees found in any religious text and if you aren't following every single one of them who are you to judge someone who is breaking one of them?
4. Studies show that children do better when raised by one man and one woman:
This study, which is often brought up by groups like "focus on the family" (side note: organizations with the word "family" in them tend to be very bad for families) as a reason to stop gay marriage. What these studies usually look at is the presence of two parents versus one parent in the home. I assure you that Rosie O'Donnell's kids or Ellen DeGeneres and Portia deRossi's kids will have a fantastic life and will not have any socio-economic issues despite the absence of a father in the home.
If you have or have heard any other reasons why two men or two women should not be wed please let me know. I'm sure we can debunk them as well.