Iced: Source that it was because of Christian persecution? See the Gemara in the end of Yevamos which says that it’s Assur to marry a second wife without the first’s permission. The Gemara in Pesachim also says that two wives is a bad idea. It’s fairly clear that the vast majority only married 1 wife even in the time of Chazal.
Also, that’s a lie about Mormons. The Mormon Church completely outlawed polygamy at the turn of the 20th century.