Many gay couples should not be parents. However, many single parents, as well as heterosexual couples should not be parents.
Adoption is an important institution. Unplanned pregnancies and unwanted children are best taken care of by a permanent guardian rather than orphanages and the Foster Care System; this is clearly demonstrated by countless studies of child success rates, that is educational attainment and avoidance of law enforcement.
Adding in more competition in the pool of adoption applicants can only be a good thing, so long as all parties are judged equally. Background checks, income, mental health, among other factors should be judged, not the sexual orientation or genders of the parents. It might be the case that fewer same sex couples will be as qualified, surely fewer single parents will be as qualified as heterosexual couples and same sex couples given the lack of two wage earners as well as spousal support. However, depending upon the other factors, same sex parents as well as single parents may be more competitive, in that they would be more likely to provide emotional and financial support to a child than their counterparts, heterosexual couples. Though, each couple/single parent should be judge on the same factors, there may be certain children that match prospective adopters better than adopters that may seem more qualified. For instance, a couple with medical/pediatric/counseling/psychology/educational backgrounds that are willing to raise a child with learning disabilities might be more qualified than a couple that makes more money in a business related field that is also willing to care for a child with learning disabilities.
Therefore, adding more applicants, such as gay couples will increase competitiveness of the applicant pool as well as more options to match compatibility with the child's needs.