there is actually a biological reason for that
this is the way i explained it to a buddy of mine, whose got 4 girls
male swimmers are really fast, but they tire out quickly
female swimmers on the other hand are slow, but the dont grow tired, so they can go the distance
which means there comes a point where the female swimmers catch up and pass the male swimmers
so the only determining factor is the distance they have to swim
find a man with all boys, short distance
find a man with all girls, long distance
now, think about what determines the distance...and that is why you have all girls