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