The one argument that could win me over in regards to same-sex marriage is that a child needs both a male and a female for parents, that a child not reared by a male and a female is necessarily in deficit because and only because of the lack of either a male or a female parent.
So far, the only "evidence" provided to support the argument is purely theoretical and is not supported by what is actually happening in real life.
Those who use the argument often act as though it would be some major experiment, far too risky to make, digressing from the "traditional" family. They act as though there aren't (and have never been) any children whom we can observe and who are reared by monogamous same-sex couples.
When they do recognize such families, it's almost always with an attitude of "sure, they're great families, but [theoretically]..."
If you want to convince me to not support gay marriage, then show me hard evidence that such a marriage is necessarily less suitable to raise a child than heterosexual marriage -- not theoretical "evidence."
Furthermore, you won't be able to convince me by dangling an eternal carrot in front of my face. You won't be able to convince me by using your standards or morality (as an unrelated example, you cannot convince me not to shop on Tuesday because it is a holy day for you). You won't be able to convince me as a soothsayer.
Again, you will only be able to convince me by showing me how children being raised by homosexual couples are hurt by not being raised by heterosexual parents.
The data is there to observe; there is no experiment; no guesses have to be made.