Google it dude.
http://en.wikipedia.org/wiki/Intersex
Sex isn't binary. There's a lot of middle ground. She falls into that middle ground. There's not really a good place for intersex people in sports.
I'm pretty sure the IAAF does require intersex people who have high testosterone levels to undergo hormone therapy if they want to compete as women. Thus it's reasonable to believe that they expected this out of Caster. I don't see any reason this should be publicized; the whole ordeal was extremely embarrassing for her to begin with, and being forced to take hormones that make you feel different, look different, and probably run slower just isn't a fun experience. Does the name Alan Turing ring a bell to you? (Obviously it's different in that for him it was the government forcing it on him against his will, whereas Caster can just choose not to participate, but I'd venture to guess that it was such an uncomfortable experience that she chose the latter).
Gender on the other hand is a mind-numbingly easy concept to understand. If you identify as female, your gender is female. If you identify as male, your gender is male. The fact that Caster has identified as female for her entire life means that her gender is female. So calling her a man is extremely offensive and ignorant. I'm surprised she has been this strong, because if something like this were publicized about me and people treated me this way I probably would've killed myself like a month after the whole ordeal.