As I understand it, all prospective adoptive parents, whether married, single, or partnered, have to undergo extensive home visits and studies and get references and recommendations.
If there is a shortage of adoptive homes and a surplus of children available for adoption (including hard-to-place children) then I think it is our duty as a society to allow all those who pass the home studies to adopt. Where a potential parent is single or part of a gay partnership, the home study should see how the applicants plan on providing loving role models who are of a different sex than the parents.
As in interracial adoptions, where white families are asked whether they will help their child to establish his/her racial identity through their own interracial friendships and community relationships, gay applicants should be asked whether they have a network of heterosexual friends who will help their child, if straight (and the odds are likely that he/she will be), to feel that sense of cultural identity as well.
Gay adoptive parents and single unattached adoptive parents may not be "ideal" but they are certainly more ideal than insecure years spent in foster care.