Honestly it'd be really fun if people could chose which gender they want to be and change it in a reversible manner anytime. I think then all the misogynist could try to out on the shoes for a day or so and learn how others treat them.
Although if that happens people probably wouldn't really care about it as much, they'll probably only change for reproductive reasons.
Reproductive biology suggests that everyone is female for the first time period after conception, since male characteristics (like genitalia) do not develop until later. (Although from cursory reading that seems to be contentious, there is a lot of semantics involved in both sides of the argument.)