There is a technical report by the Committee on Psychosocial Aspects of Child and Family Health at the American Academy of Pediatrics (Perin et al, 2002) which looks at all the scientific literature concerning homoparental upbringing, and they concluded that:
The small and nonrepresentative samples studied and the relatively young age of most of the children suggest some reserve. However, the weight of evidence gathered during several decades using diverse samples and methodologies is persuasive in demonstrating that there is no systematic difference between gay and nongay parents in emotional health, parenting skills, and attitudes toward parenting. No data have pointed to any risk to children as a result of growing up in a family with 1 or more gay parents. Some among the vast variety of family forms, histories, and relationships may prove more conducive to healthy psychosexual and emotional development than others.
They also concluded that:
Although gay and lesbian parents may not, despite their best efforts, be able to protect their children fully from the effects of stigmatization and discrimination, parents’ sexual orientation is not a variable that, in itself, predicts their ability to provide a home environment that supports children’s development.
Perrin, E. C., & Committee on Psychosocial Aspects of Child and Family Health. (2002). Technical report: coparent or second-parent adoption by same-sex parents. Pediatrics, 109(2), 341-344. doi: 10.1542/peds.109.2.341