Polygamy
Polygyny
noun, の adjective
One husband, many wives. Sounds a bit sketchy. This is polygamy.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Just note that the 一 gets changed to いっ and the 夫 gets changed to ぷ from ふう for some reason. You can imagine that this is because when you are the type that has many wives, you're probably the type who makes them clean up your poo for you as well.
たとえ一夫多妻がOKになっても、お金がなければ少子化は止められないでしょ。
Even if polygamy were to be accepted, without financial stability, it's unlikely that we could halt the declining birthrate, right?
え、日本で一夫多妻をしてる人達もいるんですか！？
Oh, are there people in Japan who practice polygamy?
チョウチンアンコウやモリアオガエル、ミツバチなんかは一夫多妻の反対で、一妻多夫なんですよ。
Chimera anglerfish, Japanese tree frogs, and honey bees, for example, have a mating system opposite to polygyny, called polyandry.