Both polygamy and polyandry are against the laws of nature. In polygamy, one man can have many women available for him but many men would be left without marrying a woman because the ratio of men to women is nearly 1:1.
So, rather than supporting polygamy, men should oppose it. Similarly, in polyandry, a woman would get many men but then there would be many women who would have to remain single. Therefore, women should actually oppose polyandry.
In both polygamous system and polyandrous system, the need of prostitution and adultery will increase as there would be many unmarried men in the first system and unmarried women in the second system who would need to fulfill their sexual desires.
Hence, under the present peaceful world where there are not many wars, monogamy would be the best system for both men and women to adopt. Of course, in the olden era because of the massive deaths of men in so many wars, polygamy had become a desired system but it is no longer so.
However, as more men and women live away from their homes, prostitution of both males and females would be required to fulfill the need of these people who are away from their homes. While I do not support prostitution but since it is the oldest profession in the world, I would call for legalisation of this system so that medical check ups are conducted on prostitutes and their clients and those who suffer from HIV/AIDS or other sexually transmitted diseases could be prevented from having sex with other persons and in turn infecting them.