Well, as far as we know, the geneseed acts off of testosterone in some unknown way, making it limited to the male half of the population. Females can be augmented, but not using the Adeptus Astartes genetech.
EDIT: In reply you your ninja skills, the problem is, the Imperium doesn't know how to do anything like that. Both science and technology are a religion to them, not a logical action. They wouldn't be able to mess with genetics because they don't have an instruction manual from more than 10,000 years ago or a Female Marine-o-Matic 9000 STC.