Usually, this is followed by how we need to have more social programs to support single-head-of-household families. e.g., better access to day care and summer program opportunities to the single head of household can earn more income; more flexible workplace policies; enhanced opportunities for telecommuting and distance education, etc.
Making this into an argument against same-sex marriage is not only unjustified, it is contradicted by the data. The data for the problems of single-head of household families tends to be rooted in economic issues and economic instability. I am not aware of any studies showing that two-parent same-sex households vary significantly from two-parent opposite-sex headed households. Mind you, I'm not aware of any studies the other way. Be very interesting to see studies.
But it is irritating to see important social research wrt single-parent-headed households taken out of context like this.