Just picture a USB-A connector. You know the metal plates on the tongue? Put metal plates on the underside as well. Connect each plate on one side, to the plate on the other side, but at the reverse end of the tongue. There you go - passive reversible.

Passive reversible USB-A is actually rather common, as a quick search will show; I first noticed this on USB drives (double-sided tongue), but cables are available with those connectors too. The reversible females look quite a bit more fragile than the males; the latter is simply a double-sided tongue, while the former has the contact block thinned to a double-contacted septum.

Reversible micro-B is also a thing, I recently bought such cable.