Purdue cites the "pollutiing" influence of semen (and bodily fluids in general) as the reason for this.
I'm not a big fan of using the word "polluting" here, as it implies a certain connotation.
Polygyny was widely practiced throughout the region, although the permission of the first wife was required.
Divorce existed and could be initiated by either party.
While such activities were acceptable in Cherokee society early on, it was a common complaint among the Euro-American missionaries that were trying to establish schools in the area.