But polygamy deserve to be decriminalized on the entire Earth because of the low rate of childbirth around the globe, especially on the civilized countries (ex. Japan, S.Korea, Europe etc.) .
The world population is growing rapidly as of now and so it is not one of the valid reasons to legalize polygamy (but IMO, it should be legalized to respect personal choices).
Also, in the European Union and the United States, the population is growing through immigration, despite their low birth rates. The case of Japan is different. They have turned asexual.
Right, but sometime, immigrate only wrong type of people, and this is not a good deal for the country who accept them.
Maybe we shall have less immigration and more natural childbirth growth. Polygamy is one of this ways. I mean we shall not force people into polygamy, but we shall open this option, like we did it for lesbian and gay people.
One man and more woman will have a bigger budget for growing properly more than 1-2 kids.