Bringing in "children can't marry" is quite the misdirection on your part, since they're universally seen as not having enough maturity for a host of things, not just marriage.

Edit responding to your new last paragragh: Regarding the parents of the child identifying as transsexual, why specify that they're adoptive lesbians? You're already stacking the deck. Lesbians can have natural children, and straight people can (and do) encourage transexualism in their child. And I don't mean 'can' in terms of finding an outlier and pointing to it as representative.

I am the natural child of a lesbian (born in wedlock to a man), I have helped adult transexuals come out at their own pace, and only two months ago I was arguing against a friend who was glorifying a straight couple in the news who were encouraging transexualism in their six-year-old daughter. I am straight myself, as was my friend I was arguing against, just like the parents we were arguing about.

So it drives me nuts when folks make an argument like you have: oh, it's not enough to decry the action on its own merits, I also have to demonise the people around the action in a way that reinforces inaccurate negative stereotypes. "The people who are fucking our kids up are these 'really weird' couples". Bullshit.