Why can't a girl marry two men at the same time since ancient times?

1, the arrangement of nature, the ratio of male to female is basically1:1; 2. The imbalance between men and women, such as war and human intervention, while marriage and sex are selfish. In the case of more men or women, there will always be people who can't get the pity of the opposite sex and create unstable factors for society. Therefore, since ancient times, a woman can't marry two men at the same time.