Probably the clearest thinker I've seen on gay marriage is Dennis Prager. I can't say it better than him, so I'm linking his column here.
Then, today he posted another column after Rob Portman announced that he had a change of heart regarding gay marriage when he learned that his son is gay.
The key is to make decisions with the mind rather than the heart. "The heart is deceitful above all things, and desperately wicked: who can know it?" Jeremiah 17:9
As emotional beings, I don't think we should completely rule out our feelings, but we need to weigh the consequences of societal changes carefully. Do we really want a society where we marginalize traditional marriage? What happens to our children? Why haven't previous thinkers pressed for gay marriage?