besides, if you want to take an evolutionary look at it, it could be argued that homosexual couples should be allowed to adopt more so than heterosexual couples:

there is homosexuality in animals. One explanation for why this can occur is that the homosexual individuals will help siblings to rear offspring, thereby increasing the chances of those offspring surviving and helping to spread a portion of the genes held by the homosexual individual.

If you accept this explanation, or accept that it may have some role as to why homosexuality persists in the animal kingdom, then it is a small step to the idea that a homosexual couple adopting children is exactly what nature intended. Of course this last line should not imply that there is any intentionality behind natural occurrences.