Perhaps you think there is a pareto superior alternative to the society Vonnegut envisages, but it is hard to guarantee that. If you had the lowest initial endowment of talent, you might prefer total equality to any alternative.

For what it’s worth, I both agree with Mankiw (the tax system doesn’t conform to any consistent set of equity principles) and you (if such an equity principle existed, it would makes sense to tax more observables than income).

Rule of thumb I would guess.

With the women men thing there is an additional issue – choice. Although a person cannot choose their height they can (to some degree) choose their involvement in the labour market. If women on average are less involved the labour market it might be more important to ask why – after all what happens if we tax women less (given that their labour supply is more elastic) and then the household structure changes to take advantage of this tax structure? In that case we would end up chasing our own tail moving the tax around.