At that stage there is not enough hormones in the plant for it to be able to properly decide what gender it will be.
According to some experts I have read and stuff I have researched, including the ever famous Jorge Cervantes(that guy knows his crap), seeds gender are not pre-determined only by genetic factors, but, are influenced by enviornmental factors, such as heat, light, wave lengths of light, (ie: a yellow light is said to encourage male hormone), etc etc.. I've read that at germination PH should be just a tad higher, in the 7.5 - 8.0 range, for the first week or two of germination, it will help influence towards female.
One interesting theory that I read and I have recently begun using is Red stem vs white stem at "first leaf" or the "cotyledon stage", with some success.... red stems are male, white are female.... this method has worked for well for me in minimalising males but, it's by no means ABSOLUTE.
One thing that's cool is.. a female can also be a male...
but, a male can never be a female.