In what way is it detrimental to children to be told they can make their own decisions?
It is detrimental to children because from the time they are born we are immersing them in a society that says pink and dolls and ponies = girls and blue and cars and dogs = boys.
We are also telling them, factually, that vagina / flower = girl and penis / willy 
= boy.
But three year old Becky doesn't like dolls and she certainly doesn't like pink.
FINE.
We need to be teaching a very confused becky who thinks she is a boy (because she is immersed in a society where blue and cars and dogs = boy) that becky can still be becky and like blue and that gender stereotypes are exactly that - stereotypes.
What is harmful is telling Becky that because she likes blue and cars and dogs that she might be a boy.
That's the detrimental part. The inappropriate part.
And, since you keep wanting to use gays and lesbians to further your point (they must get SO sick of being dragged into this so apologies) here is the equivalent:
Three year old Jack thinks he might be gay.
What we do is tell him that's fine, love comes in different forms and sometimes it's a man and a woman, or a man and a man, or a woman and a woman. And we let him grow up and discover what he is.
We don't go any further than that, do we? So why do we have to go further with Becky?
And for someone complaining about derailing - the quickest way to do it is to put words into other peoples mouth so they need to explain themselves....