Because there are people who pretend to be animals, objects, and even fictional characters.
Anyways, I don't necassairly have a problem with feminine boys or masculine girls, they can act how they want for all I care, it's the fact that they think they're of the opposite *** when their genitalia, and DNA, are what decided that. The term man refers to a male human, as does the term woman refer to a female human.
Is a transman a female human? No.
Is a transwoman a male human? No.
Then that's that. You can't just change the definition of the word. It's like if I called myself a plant even though, by definition, I am not, and no matter how much I insist I am and cry to the government to ban the big meany bigots to stop making fun of me, it won't change the fact that I'm not a plant.
The whole of humanity can call me a plant, with me dressing in green, coloring my skin green, bathing in the sun and taking up whatever from my rear end, with my legs in the dirt, I still will be nothing more than a human.
You can't change the way the universe works, and the universe, or God for me, has declared that men are men and woman are woman, and no amount of emotional turmoil over such things will change that, even if all the world claims as such.
Think of it like this. Remember when the whole world thought the Earth was flat? Did that make the Earth flat? Nope.
Likewise, even if the whole world thought a transwoman is a real woman or a transman is a real man, that won't change the fact that they're not. Words can't change reality.