Why are only men allowed in?

The United Grand Lodge of England is a fraternal organisation so only men are allowed to join. However there are many lady freemasons across the world. Some belong to women orders of which there are two main in England - Order of Women Freemasonry and the Honourable Fraternity of Ancient Freemasons. Others belong to the mixed, known as “Co-masons”, which admit both men and women. Often lodge venues are shared in London by all.