Because it is a deviant lifestyle that is detrimental to society. Because it is a lifestyle choice that leads to the exploitation of women. Because it distributes the allotment of time and resources of the single father to the many offspring and leaves them often poorer and with less resources than their counterparts in traditional two parent relationships (which society has recognized is the ideal).
Not that I support it but consider the following if we use your explanation.
Explain this: Wouldn't one man, two women who each have two children to the "husband" be better off then one man, one woman with four children? More resources for the same amount of children. Also shouldn't we then outlaw divorce and second marriages?
"Because it is a lifestyle choice that leads to the exploitation of women."
And children. Many wives are taken at a very young age. I even saw where one guy was married to a woman AND her daughter from a previous marriage.
Yeah, that just good clean American values at work.