Agnosticism is the view that the existence of God, of the divine or the supernatural is unknown or unknowable. Another definition provided is the view that "human reason is incapable of providing sufficient rational grounds to justify either the belief that God exists or the belief that God does not exist."
Muslim man is not allowed to marry an agnostic woman. Marriage is an establishment of shared life between man and woman which must be based on principles of Faith in Allah, The Glorious.
Marriage is not just for fulfilling sexual desires, but to build a shared life based of Faith which is the main condition of harmony.