if we are considering wiki* sources legitimate, then let's visit the wiktionary page for "binary" that the Wikipedia page for "Binary" links to: [1]. The first definition is:

    Being in a state of one of two mutually exclusive conditions such as on or off, true or false, molten or frozen, presence or absence of a signal

So, the "two-sexes approximation" is binary :)

1: http://en.wiktionary.org/wiki/binary