is it because women haven't got the time to sit over a piano drunk for three weeks
Ooh, get
you. ;-) There have actually been a fair few female composers over the years, arguably the best known being Clara Schumann (1819-1896), wife of fellow composer Robert, and 12th century abbess Hildegarde of Bingen. Several well-known male composers had composing wives (Alma Mahler, or the aforementioned Clara Schumann), sisters (Maria Anna Mozart, Fanny Mendelssohn) or daughters (Imogen Holst). I guess their comparative lack of fame is purely down to the rather chauvinistic societies in which they lived - art, literature and music being seen as something men did.
Good old wikipedia, as usual, has a lot more info:
http://en.wikipedia.org/wiki/List_of_female_co mposers