Wanting to be a girl when you are a boy is not normal. As far as being a mental illness I don't know. I think society has a lot to do with it. As you normalize a certain type of behavior, it becomes, well, normal. It becomes more generally accepted, even encouraged, and you'll get more of it. Like in ancient Greece it was considered a status symbol and normal to have a boy sex toy. I lived in a time when girls rarely swore and when they did it was a head turner. Now it's just part of the everyday vernacular.
One of my brothers is married to a very Liberal woman. One of their daughters, who is 3 years old, is a very rough and tumble Tom-Boy type. Anything physical: running, jumping, climbing, wrestling, loves hitting the mits (my hands)... She's the one that I posted a pic of doing situps and when she got to 10 reps I congratulated her, but she just replied "Two more." Not one more. But two more. My heart melted.
Once we were looking at clothes and she picked up and looked at some shirt that was for boys. Just randomly picking things up and looking at it like all kids do. And then I took it from her and said, "You don't want that. That's for boys." She then said, "But I like Spider Man. I can be a boy." I kind of laughed and said, "Um, no you can't but we can find another shirt with Spider Man." Then her mom chimes in, "You want to be a boy? You can be a boy if you want to." I immediately went into culture shock panic. I'm like "Noooo! What are you saying? Gender is determined at birth! You can't change DNA!" I was actually shielding my niece and blocking her from her mother. It kind of took her off guard and furthered her subtle dislike of me.
You read about all this shit going on in the world and shake your head at how crazy everything is getting. But, man, when it hits you at home alarms start blazing.
I wonder if any parents nowadays tell their son to act like a man or their daughter to act like a lady. Or if they even know what that means.
I'm so glad I'll be dead when all this goes full steam and the becomes the norm.
"And you knew what you were then,
Girls were girls and men were men.
Mister, we could use a man like Herbert Hoover again."
-- Glenn Miller, Those Were the Days
Forcing out those last couple of reps Arthur Jones' style.