The fight over whether GLBT couples should be allowed to adopt is heated and has been going on for quite awhile. Arguments against gay couples adopting range from concerns about the children getting teased and bullied, to accusations of sexual abuse and immoral behavior. Those in favor of gay couples adopting often say that it is just prejudice standing in the way and that two parents of the same sex can provide a stable home to a child just as well as a heterosexual couple can.
There have also been discussions over whether allowing a gay couple to adopt a child will harm the child emotionally and mentally and whether it will influence the child to become gay themselves. If you look at the evidence though, studies don't really support this theory. Most studies show that gay couples raise about the same number of heterosexual children vs. gay children as traditional couples would.
As for whether being raised by gay parents affects a child emotionally and mentally, that is hard to qualify. Certainly children who have gay parents may be more prone to being bullied and teased if classmates know about their family life. However, you could say the same thing for children of black parents or hispanic parents. I'm not sure that kind of prejudice is a good reason to base a legal statute on.
There have been children raised by gay adoptive parents who have been outspoken about how much the experience affected them negatively.