It should be up to the parent(s) of the kid being put up for adoption, or the adoption agency. They should be able to specify whatever requirements they like..
This +1000.
Ideally, adoption is a private transaction. People don't like to think of it that way, because it seems to place monetary value on a human life and make babies a commodity. These same people don't have as big an objection to "fertility treatments" which design babies and implant them, and usually lead to rejection of some of the implantation attempts. Those clinics are not free. People are paying to have babies all the time.
A mother putting a baby up for adoption should be able to choose an agency she feels comfortable with. This might be, for instance, an agency that promises to place the baby in a racially-consistent home ("black" babies with "black" parents), or maybe it's religion that's important to the mother, or how about an agency that caters to active parents who will ensure the child doesn't grow up a couch potato? There would be, I am quite sure, "all comers" adoption agencies who simply accept any child and attempt to place them with any willing home. There might be mothers who don't care where their baby goes to, and those agencies would fill that void.
As far as my opinion? I don't think gay parents make gay children, but I do think that growing up around a behavior makes it seem more normal. Depending on the parents and the community in which the child is growing up, they may be in for culture shock once they leave it. This is true for all kinds of different life experiences, though; you could grow up in an "ethnic" neighborhood and then go to college in White-As-Anything, USA, and you'd be pretty confused by some of the attitudes there.
I think that having parents who are going to care for the child in question is the most important thing, and I would hope that all those agencies I mentioned as hypotheticals would have their own standards for such. Some gay parents WOULDN'T qualify to adopt a child, I am certain. Some heterosexual parents certainly don't. If your home is dangerous (again, determined by the agency), you have no means to care for the child, you have no time to care for the child, you run a meth lab in your basement (they do explode at times, and the fumes aren't exactly healthy over the lifetime of a child)... I'm going to have to think most agencies are going to deny you, regardless of any of your other traits.
