Exactly. Using the analogy of boxes one could think of "items" in the box as for why it is so wrong.

The masculinity box holds assertiveness and confidence. The femininity box holds empathy and caring. Never shall the contents of the boxes meet. The content of the boxes are given to persons who's biological sex align with the box label.

No sharing.

So if you're a guy and your dog gets sick you're not allowed to take care of it.

That's stealing. You using stuff that's not from your box.

Cheater.

If you're a gal and your kid runs in front of a car you can't save it. Too assertive. Not allowed. You can scream helplessly, "My baby!" though. That's allowed. It shows concern. Just don't you fucking act.

And that's an extreme but accurate snapshot of just how fucking ridiculous these gender boxes are.