All men and women used to wear it in some form or the other. Now a days, most men wear it only during worship and ceremonial occasions. Many women also do not wear it all the time.
Most experts would simply state the origin as unknown. I dare to suggest an origin. In the books on the lost continent of "Atlantis and Lemuria", one notes that the people of "Lemuria" had a third eye on their forehead. The South Indian mythology also points to such a lost continent. The dot on the forehead is a lot more popular in south than in the north. Both among men and women. Most human beings can feel the presence or absence of a infra red light at the center of the forehead. The popular God Shiva, who is timeless and formless has a third eye on his forehead. The dot signifies the third eye.
Now a days, it is more a beautiful beauty mark. Even many American pop stars wear it sometimes.

