This isn't really directly related but is somewhat indirectly related to the current situation we are talking about here.
There is a theory of a gay gene way back in prehistoric man where there was a dominant male and, since biologically, males are supposed to be fighters, or to say the least, more physically capable of hand to hand combat than women.
Homosexual individuals would be left with the females as a method of protection.
So, if this is a kind of adoption because if you trusted a person not to sleep with your daughter, aunt, stepsister, sister, or in this case, an adopted child, then, it was seen as something which could be beneficial to the group.
Video is seen below:
[video="youtube;MHDCAllQgS0"]http://www.youtube.com/watch?feature=player_embedded&v=MHDCAllQgS0[/video]
So, in a sense, homosexual individuals were and still possibly could be seen as adoptive individuals, if you have it, "nannies" to people in need of a home.
I am not trying to apply this to such a full extent back then, but, the adoption part could be seen and brought back (well, when I say brought back, I mean as it was then without the mother/father role model BS issues). How it disappeared I will never know.
Although, these are all theories and no proof exists as of yet.