Actually, they really aren't, anymore. Most nations that have legal polygamous marriage have certain cultural norms or civil laws which need to be followed. Most Muslim nations have it on the books, but it is still pretty rare because it is a cultural artifact of a bygone era. In Indonesia, for example, a man needs to prove that he is financially able to take on another wife, AND his first wife must file her written permission with the courts before the state will allow it.

The two person paradigm for marriage is pretty much universal in all religions, save Islam, certain types of Tibetan Buddhism and Taoism, some small Sephardic Jewish Communities, and the FLDS Church.

And even then, the "mainstream" Mormons are trying to erase it, while some other extremes are actually denying it was ever taught... By the way, is "F" in "FLDS" for Fundamental?

Yes. The FLDS are a breakaway sect from mainstream Mormons that number about 10,000 who still practice Polygamy (albeit illegally).

The HBO show Big Love is based on them.