Yes, in fact, several posters decreed that a transgender can only be considered to have transitioned if s/he has ALL the necessary surgeries. So posters WERE in essence taking it upon themselves to decide what defines a male or female.
Of course, you are entitled to your opinion. Personally, I am cautious about forming opinions on subjects about which I have no way of becoming an expert. I really don't see the harm in allowing people to define their own sex and sexual orientation.
Why must they conform to your (rather narrow) definition?
"Born a woman, die a woman" begs the question: is sex primarily determined by external genitalia, internal hormones, chromosomes or psychology? Those most affected seem to say that the psychology predominates.
I see no reason why you or I wouldn't take them at their word. Because again, where's the harm?