There is no good reason why anyone would disallow gay marriage or adoption for gay couples. In fact, in many instances, it would seem that gay or lesbian couples are far more fit than heterosexual couples to adopt children. That being because of a better education, job, or simply being open-minded.
However, the reason some of us don't like the idea of gay people adopting children is because it conflicts with the default norms of our society. People are usually resistant to change, and we like to hold and maintain good old fashioned values because they make us feel safe and assured. It's more to do with our natural human psychology than it does with sheer prejudice, sexism, homophobia etc..
We simply crave fammiliarity and fear the obscured and the mysterious. At least most of us do.