Strange. I know a lot of people who are supportive of gay marriage, but not gay adoption. But the other way around it's kind of rare. Mitt Romney is really something.
I think I find opposition to gay adoption, in principle, even more hateful and disagreeable than opposition to marriage.
Americans tend to take as granted that anyone who isn't physically abusive is entitled to raise their children however they prefer.
EDIT: Adding the weaselly qualifier "within reason," before someone calls me out with a counter-example.