I think that I agree with you, which is itself a wee bit miraculous!
I think that the issue is the environment that the child was raised in. A homosexual couple who displayed outward physical signs of affection may well be arguably slanting the kids societal development...so says one argument. (I have no proof,statistically, though)
I suppose that it could be argued that a heterosexual couple who showed abnormal behaviors would be just as harmful to the child, however.
Lets say that the issue was adoption by single people. Sexual preference would not be an issue. Behavior and role modeling would be the issues. Stable home environment, ability to understand and be responsible for the child, etc etc.
I am not sure that I favor forcing the churches to adopt a standard unlike their creeds, but perhaps if they are too rigid they should not be in the public child adoption business.