Human dignity of woman does not allow her to be with more than man at the same time. It is not only in Islam, but in all main religions that woman is allowed to marry one man at the same time. More than one man at the same time means destroying the dignity of the woman and shaking her self respect. Obviously, if a woman is allowed to marry more than one man at the same time, the society will lose trust in the belonging of the children to their father as no one will know who is the real father of the child. That itself is a big damage to the human sonciety.