Most societies throughout history depended upon men for the protection and defense of their existence. Before projectile arms, females were simply considered weaker in battle, as well as submissive; as nurturers, they were also seen as less likely to finish off an injured enemy. A male exhibiting female behavior in any way would be suspect to submitting to an enemy quicker than a 'normal' male, so he would be considered a weak link in the chain of protection that society, and as such, wouldn't be considered useful for anything, but using up food and shelter resources. Women see feminine men as potentially not willing to defend those women from attack. So women don't see us as potential mates to have children. Then of course the sexual desire is gone, so they have no use for us either. Hence the propensity for our societies to consider mtf behavior a taboo.