I personally support gay marriage (I'm gay and married, what can I say?) I have heard many arguments against gay marriage, but they usually don't hold up very well to reasoned argument.
I'm hoping someone can come up with a strong argument that actually uses reason. Quoting a passage from the Bible, and telling me that this is the way God wants it, doesn't count as an argument using reason, by the way...
I do not necessarily agree but here is the argument. The purpose of giving a married couple special rights under the law is to make it easier for them to raise children and pass their physical wealth and property on to the next generation. This is assuming that the partnership is a lifetime commitment and that children are raised (these may not be valid assumptions). A gay couple can not directly produce children. This is not to say that they do not raise any children, just that biologically they cannot produce children. The main purpose of marriage in Western culture is the production of children.
Where the gay community is falling short is in their lack of appropriately enlightening the general public about their problems having long term commitments without legal rights. No human alive is without moral sin. Some do not recognize that intolerance is a sin.
What complicates the issue in today’s American society is that there is still no scientific proof as to the reason people are gay. The general population fears that being gay is a learned response to environmental factors has little to do with genetics. Many assume that gay couples would raise gay children. This would defeat the purpose of marriage in their eyes.
Those that wish to make a moral issue out of it tend to be intolerant to others who are not part of their own subculture. Those who are misinformed tend to have no understanding about the legal issues with shared property, hospital visitation rights, insurance problems and others details of which you know but that they have never considered.