The aforementioned statements from the APA are from 2005. Since then the UK has passed a law that allows same-sex couples to adopt. As a result, there has been more data coming out supporting a pro-same-sex parents stance. One example of this is the following study from Cambridge University: http://www.cam.ac.uk/research/news/ive-got-two-dads-and-they-adopted-me

This study finds that “In-depth research into the experiences of adoptive families headed by same-sex couples suggests that children adopted by gay or lesbian couples are just as likely to thrive as those adopted by heterosexual couples. It also reveals that new families cope just as well as traditional families with the big challenges that come with taking on children who have had a poor start in life.”