Unfortunately, biology and the rights of the child run counter to the ideals of homosexual couples. Before I explain what I mean, for the record, I support adoption for same-sex couples and I also don't believe that anyone technically has any natural right to know who their parents are. It's a good idea, but not a necessary right like say free speech.
Anyway, in the past, part of the operating ideology behind adoption and sperm/egg donations has been to take great pains to conceal the identities of biological parents or donors. There's a long standing assumption in our culture that implies there is something shameful about adoption. I'm not sure how well it reflects actual social behaviour, but there is at the very least the meme of parents hiding adoption from their children and the horror of revealing it to them.
Attitudes have changed over time. Our government has made it easier for children and biological parents to find each other. Children believe they have a right to know who their parents are, and some are even looking for their sperm donors. When it comes to these things, we often forget that there are real human beings brought into this kind of contract without their consent, and when they grow they have questions and reasonable demands.
Unfortunately for same-sex couples, we still can't produce children without a male and female, so no matter how you do it (adoption, donors) you're going to bring third parties into the equation. The ethical questions involved are the same ones posed opposite-sex couples adopting, using donors etc.