That's a bit of a generalisation, there are several Muslim societies where polygamy is legal yet there are more women than men in the workforce, with their own businesses and studying at university level, even more so than in the US. Even in those societies viewed more negatively, this negative viewpoint is often based on false stereotypes. Talking about 'East and West Africa' like its one homogenous blob of a country is insulting to say the least. Not every African country where FGM is practiced has polygamy and not every African country where polygamy is practiced is FGM found there.
Some societies where polygamy is or was common are strongly matriarchal, look at the Navajo. Although its no longer online a well known Navajo feminist had a blog that was very pro-polygamy and she rued the fact that polygamy was far less common amongst her people now as it did have many benefits for them. Anyone who knows how Navajo society works knows its the mothers and grandmothers who are asked for a girl's hand in marriage and they have much of the power financially as well. I don't consider more traditional societies than ours to be backwards by default nor do I consider the marriage vows made up by Church leaders in a few western Christian churches to be a model of marriage that all humankind should abide by, that is very imperialistic in my opinion. Just because someone has a different view of a normal model of marriage to you or what is commitment to a person in a relationship doesn't make it wrong. As for diseases etc far higher risk of those with serial monogamy and men sleeping with prostitutes, most of those in polygamy have on average slept with less partners than those in monogamy. Xx