Why the binary option for gender on profiles?

[RenCutypoison]

brendude13 said:

RenCutypoison said:
On Belgian's official documents (and probably other countries too) it's written either X(male), Y(female) or Z(for undefined).

I tought it was funny until I read hurou musuko

Which idiot came up with that?

I. I like to think it only happens to the best of us.