The reason women cannot become Freemasons comes from two Landmarks of Freemasonry. The Landmarks of Freemasonry are unwritten laws that form the basis of every Grand and subordinate Lodge constitution. The Landmarks are the foundation on which Freemasonry stands.
LANDMARK EIGHTEENTH Certain qualifications of candidates for initiation are derived from a Landmark of the Order. These qualifications are that he shall be a man, shall be unmultilated, free born, and of mature age. That is to say, a woman, a cripple, or a slave, or one born in slavery, is disqualified for initiation into the rites of Masonry. Statutes, it is true, have from time to time been enacted, enforcing or explaining these principles; but the qualifications really arise from the very nature of the Masonic institution, and from its symbolic teachings, and have always existed as landmarks.
and
LANDMARK TWENTY-FIFTH The last and crowning Landmark of all is, that these Landmarks can never be changed. Nothing can be subtracted from them-nothing can be added to them-not the slightest modification can be made in them. As they were received from our predecessors, we are bound by the most solemn obligations of duty to transmit them to our successors. Not one jot or one title of these unwritten laws can be repealed; for in respect to them, we are not only willing but compelled to adopt the language of the sturdy old barons of England - "Nolumus legen mutari."
A woman cannot, for these reasons, ever become a Freemason. Any organization that purports to do so is Clandestine and not Freemasonry.
This in no way means we think of women as less than men, or that the OES is subserviant. This is just the way it is. To abandon the Landmarks would be to abandon Freemasonry.