Well, it's because of the way the Leftist Progressives address the rest of society about this issue - it's fingerpointing, namecalling and accusations - as if acknowledging biologicial sex is now to be equated to racism and hate speech. Only "bigots" deny that a transwoman is a "real woman."
And, then the entire theory doesn't make a lick of sense - one says, "okay, what's a real woman? Define woman." The answer is "a woman is someoe who identifies as a woman in terms of gender." Which of course, is nonsensical, because we are still left with not knowing what a "woman" is - what are we identifying as when we identify as a woman? It's whatever one's personal definition of a woman is! Because, we know the traditional qualities associated with being a woman are just social constructs, and there is no such thing as the look, fashion, behavior, appearance, or feeling of being a woman - a woman can be feminine and delicate - or a woman can be tough and masculine - who ware we to say? Caitlyn Jenner became a woman and dresses and tries to act like a "traditional" woman - but there is no reason Bruce Jenner couldn't have shaved his head, and put on cargo shorts and said "this is how I express myself as a woman," in his deepest baritone voice. Because, being a woman is just a social construct, and the definition of a real woman is "someone who identifies as a woman." Surgery, hormones, and changing fashion/styles is not necessary to "be" a "real woman," according to the Left.
It's a jumbled mess of meaningless terms that boil down to "whatever I say subjectively is what it is, and I can change it whenever I want, AND you have to accept it -- all of you have to accept it - and if you don't you're a bigot." That's what it boils down to. If Caitlyn Jenner said tomorrow - you know, I realized I'm actually a man, and I had this epiphany about that. I'm not going to change back to my old Bruce Jenner appearance, and I'm changing my name to Carl Jenner, keeping the dresses and makeup, but now you call me "he/him" instead of "she/her," according tot he Left we'd all have to cater to that, and the media would have to make sure it refers to Carl as a he, and they'd be bigots if they "dead name" him Caitlyn, or Bruce, and they'd be bigots if they dead-gendered him and referred to him as her, even though he wears a dress..... that's the logic the Left is applying.