Let's see here. Surely the majority of states in America would not ban gay marriage without good reason, right?  So let's go through those reasons briefly.

1. The majority of the population is Christian. Christianity forbids it.
- Well, our government isn't Christian and can't make laws based on religion of any sort, so that argument fails. Next try?

2. Gay marriage is not traditional. Homosexuals have never been able to marry before, so we have no moral/legal obligation to allow them to marry now.
- Well, at one point it was tradition for law to prohibit women from voting, allow the enslavement of Africans, and to keep people of different races from marrying. Tradition sucks, tradition changes. This is the story of America.

3. Children in families with gay parents will probably turn out gay.
- Cough cough...there is nothing wrong with being gay. And anyway, gay parents do not raise gay children any more frequently than straight parents do.

4. Children in families with gay parents don't develop normally.
- There is absolutely no evidence of this.