>So what we consider to be a disability does seem to be a function of what we consider to be normal.

Obviously? How could it be based on anything else? People are just much more uncomfortable with making normative statements than they used to be.

If it were a fitness advantage if you could sense electromagnetic fields, then why have you evolved over billions of years to get where you are, without it?

But wait, you do sense electromagnetic fields in the 380 to 750 nm wavelength range, and remarkably well, to great profit.

The only fitness advantage that matters for evolution is whatever gets you to pass down your genes, versus someone else not passing down theirs. If sensing low-frequency electromagnetism, or static magnetic fields, were advantageous in the context of everything else that you are, for passing down your genes, you would have it by now.

Migratory birds can sense the Earth's magnetic field for navigation; if you needed to migrate thousands of kilometers every year (due to lacking other advantages to make that unnecessary), you might evolve that.

> if I could sense electromagnetic fields, but no one would describe me as disabled for not being able to sense these fields—unless, perhaps, everyone else could.

Light is EM fields. A possible scenario is a battle at night with others having night vision equipment and you don’t. You can absolutely be described as disabled or being at a significant disadvantage.

Because, like you say, what we consider normal in that scenario is to have a proper night vision equipment.