"Problems arise when selecting a personal pronoun to refer to someone of unspecified or unknown gender (see also Mixed and indeterminate gender above). Traditionally the masculine has been used as the "default" gender in English."

It's just proper English by traditional standards to use he when you don't know the gender of the person. People take it as some kind of an affront, but those of us with English training were just taught that way in college. Source: Minor in English.

http://en.wikipedia.org/wiki/Singular_they