Both have happened here plenty of times and I haven't heard or noticed anything bad about it...
Adoption doesn't need to be forced on gender, but on the abbility to raise kids (and both man and woman can fail terribly at that...)
The % (ofcourse not of the total of children born/adopted, but of the groups) of childkillings done by Man-Woman is far higher than with Man-Man or Woman-Woman.
And why should one prevent two of the same gender to marry like a woman-man would? Does it hurt "anyone" if they do? Let them have their same rights; we are all humans after all...