There have been a wide variety of studies in recent years suggesting that children who are raised without role models of both genders tend to be predisposed to a wide variety of social and behavioral problems later in life.
Witherspoon Institute - Men Don't Mother
You cannot toss the last several million years of human evolution out the window just because it doesn't agree with your ideological views.
Frankly, how many gay couples actually want to adopt children anyway? I'd assume that they are something of a minority. In a group that only makes up 2-4% of any given population in the first place, that's saying quite a bit.