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 in America, most people don't want government intrusion. Adopting a child is a personal choice based on things such as financial resources. In some ways, it is better to adopt a child if you have the financial means to raise the child, instead of having six biological children and not being able to financially provide for their care and well being.
When you consider government subsidizes, they are urgently needed for large biological families who cannot financially support their children, an example would be the Octomom, who is now living on welfare. At least when gay couples adopt, they do so by choice and have children by choice, instead of being forced to have children.
When there are biological children, both the biological father and the biological mother are legally compelled to provide child support for the child depending on where the child lives, even if the parents are divorced. These are not based on new laws, but based on Common Law. The basis for Common Law of co-habitating gay couples would be far more difficult to prove. This progression of common law may even effect platonic roommates.
In addition, when you legalize gay marriage, you bring the government into the lives of the gay couple that effects their finances and legal arrangements. The numerous government programs will also be subsidizing gay couples along with heterosexual couples, and this will further drain the budget and increase government spending and waste. A lot of libertarians see the government as evil and meddling in the lives of straight married couples, and don't want the government meddling in the lives of gay couples. If it were up to libertarians, then no one would get legally married, even straight couples.
In the long run, Common Law will essentially decide the legal responsibilities between male/female couples.
People are in love with the idea of marriage and the romance of love it aspires; but a lot more men and some women are declaring that marriage is a good idea but not for me. In the next few years, gay marriage will not prevent the decrease of couples marrying, so therefore Common Law will end up dictating the lives of the straight couples who co-habitate with children.
