Why were woman not allowed to act during Shakespeare's time?

I assume that you are asking this question in reference to a Shakespearean play such as Romeo and Juliet.  In Shakespeare's time, acting was considered a low profession, not far above prostitution.  For a woman to be an actress was asking for her reputation to be wrecked by society's standards.  Therefore, women were not allowed to act as they would voluntarily be lowering the standards for all women of the time. In their place, young boys with high voices played the parts of women.  Also, acting in this time period was not for the faint of heart.  If the audience did not feel entertained, they threw things like rotten tomatoes or small rocks to express their displeasure.  It would not be considered seemly or proper for women to have things thrown at them as part of the cast of a play.