I think the platonic idea of "forms" actually helps to differentiate male and female when dealing with trans people. There is an "ideal" male and female form, ie. men have a penis, produce sperm, upper body strength, body hair, a prostate, etc. and women have a uterus, eggs, breasts capable of lactating, less body hair, strength is focused in the lower body, etc. These combinations of traits can be altered, but no matter how far away a male gets from the "ideal" male form, he does not cease to be a male nor become a female. The same is true of women.