Polygyny is a relationship in which a man has a relationship or marriage with multiple/many men. This could be due to religious beliefs or cultural acts, but it is not required.
Note that although FLDS* and other religious polygamous relationships tend to be polygny, they just fall under the polygyny umbrella. The specific name for these relationships are "Plural Marriages". It should be noted for the sake of accuracy that the FLDS is different from The Church of Jesus Christ of Latter-Day Saints (more commonly called Mormon or LDS but those names should be avoided,) and that The Church of Jesus Christ of Latter-Day Saints no longer practices polygamy.
Flag
The polygyny pride flag was coined by Deviantart user NonMonoPrideFlags on December 3rd, 2015.
Resources
https://www.deviantart.com/nonmonoprideflags/art/Polygyny-Pride-Flag-576036367