Even as a single woman I have been told I should be open to either gender, because there are for whatever reason more boys than girls available for adoption. The impression that I get is that girls are more "in demand" and agencies prefer to adopt to married couples first, then singles--so if the couples also want girls, and the single women very often want girls, there's hardly anyone wanting to adopt boys.
I also think there's an unspoken worry many people have that a single man adopting a girl would somehow turn into a sexual relationship. Not saying it's true--just that this is the assumption.
I have spoken to two single men recently who were adopting and asked whether they preferred a certain gender. They both said they didn't really care but suspected they would end up with boys, because they are male.
None of this bias and discrimination seems right to me--single men and single women should both be able to adopt either gender, as should married couples, and it should not matter whether one is single or married. But, from what I hear and read, it does matter in a lot of situations.
