Child of a Metal God

The obstetrician judges solely on reproductive organs, likewise the botantist (there are edge cases for both, which you gloss over) not so for people in the street, and neither can you reasonably say that they're inferring about genitals, they're inferring about gender/sex and thinking about genitals from there if at all. Fertility is a proxy, but not what the arbitrary distinction actually is, even if the arbitrary distinctions and assumptions themselves inform evolutionary processes that are fundamentally concerned about fertility. By that measure, all one needs to transition is hip pads and greasy hair. But those aren't the only factors being looked at, because social pressures and evolutionary ones have built up all sorts of standards perhaps best described as gender (in the linguistic sense of attributes/classes of nouns; since certainly other measures of fitness than simply fertility are evaluated) rather than sex per se.