We are not singling them out to allow adoption, unless by "singling out" you mean "including in the same privileged class as straight housewives, baristas, scientists, garbage collectors, members of the armed service, naturalists, and, depending on the state, convicted felons."
Seriously. Parents come in all shapes and sizes--surprisingly few restrictions are placed. Why are gay people barred from adopting when longitudinal studies show their parenting outcomes are basically indistinguishable from "normal" families?