Interesting question. I would say that no one has collected enough data to make good conclusions about the effect of homosexual parentage on children, especially in those children's eventual sexual choices. Being raised by two openly gay parents is a very recent social phenomenon, and the sample groups would be small in any case.
Secondly, if one is going to maintain that gay parentage is equivalent to heterosexual parentage, one must then say that neither mothers nor fathers are of any real, particular value. Of what value is motherhood if the mother can simply be replaced by a man? Of what value is fatherhood if the father can simply be replaced by a second woman?
Humans are not aftermarket parts that can be slapped into the same slot the original was intended to occupy. Children need a mother and a father, and short-changing them will never be right, no matter how we rationalize it.
And by the way, my lovely nursing student wife informs me that one of the predictors for anti-social disorder is not having a parent of each sex. Now, they aren't only discussing homosexual parents in that conclusion, but it is an indicator. The relational styles of the two genders work in concert to bring a child to a healthy maturity.
Homosexuals are mostly hyper-sexualized in their behaviors. If children are raised in a hyper-sexualized environment, it effects their socialization as well as their sexualization. Homosexuality is deviant sexual identity accompanied in the vast majority of homosexuals with deviant sexual behavior, (in my books, pre-corrosive psych 'normalizations'), but any home where hyper-sexualization is influencing the child would be detrimental to the development of a noraml sexual and social persona.
Also, you might want to see post 117.