Mainstream Masonic Grand Lodges rationalize the exclusion of women from Freemasonry for various rationalities. The structure and customs of modern day Freemasonry is founded from the operative medieval stonemasons of Europe. These operative masonic guilds did not permit women to join, because of the culture of the time. Many Grand Lodges are of the opinion that changing this structure would entirely change Freemasonry.
What is more, mainstream Grand Lodges stick to the masonic landmarks represented in the early 18th century, which are considered to be unchangeable. One of these landmarks determines that a woman is not to be made a mason.
Finally, in many jurisdictions masons swear “not to be present at the making of a woman a Mason” in their obligations. A lot of masons believe that irrespective of their opinions of women in masonry, they cannot break their obligation.