1. Men have more problems achieving fulfilling sex as they get older. Due to a decrease in testosterone, many men suffer from erectile dysfunction. Many men also suffer from prostrate cancer and other diseases that will cause their sexual ability to decrease. Hence, women must be allowed an honorable way of having a fulfilling sex life without being forced to abandon their first husbands.
2. Many men suffer from illnesses that prevent them from fathering a child. Polygamy allows women to become mothers even if their first husband can not father children, without getting a divorce.
3. There are more fertile men in the world than fertile women. Hence, allowing women to marry more than one man means all men can be allowed to marry. It also means that even if a woman marries many men, she can choose which man should be allowed to be a father of her children, thus fulfilling evolutional needs of only the best being allowed to procreate, without denying men of less qualities the right to marry and have sex.
4. Men being aggressive, jealous and irrational are in greater need to humble themselves, fight their jealousies and submit to somebody more empathic, cool and rational.
5. Most men can only fulfill the sexual and emotional needs of one woman within a certain timeframe. (Since most men have problems with emotions and communication, actually most men have difficulties fulfilling the emotional needs of even one woman!) Women on the other hand are emotionally competent, communicative and empathic and can easily fulfill the emotional needs of many men. A woman can also have sex with any number of men the same night, something a man can never do, even if many might wish to.
Polygamy is a sensible solution to these problems.