Users can set their gender manually in the preferences after creating an account. Lots of users, however, never bother to adjust their preferences and are not aware of the fact that they can choose the gender. This is a problem, because some software messages that are specifically at new users are gender-dependent in some languages. An example from today is the MoodBar extension messages and there are more.
This must not be forced and the default gender should remain "unspecified", but putting a gender dropdown somewhere in the registration form would be useful, because it will save the translators and and the users a lot of confusion.