There are several reasons why women should never ever accept polygyny. Most of them are pretty obvious, but I’ll state them here anyway since there are criminal, immoral people out there who still claim that women should submit to polygyny.
1. Polygyny brutalizes men and makes them indifferent, arrogant and numb regarding the emotions of women in general and their wives first and foremost. Polygyny gives men the false impression that they have rights their wives don’t have, that their rights are one degree over the rights of women, to their mental and emotional detriment.
2. Polygyny deprives women of a partner in life, and instead offers them a houseguest with sexual privileges 25% of the time.
3. Polygyny makes a mockery of the prophet since it is clearly stated in the quran that it was only allowed under very special circumstances during a certain period of time. While muslim men have used both the quran and their intelligence to oppose slavery, they have used their intelligence and their libido to keep polygyny, although both are treated with equal restrictions in the quran. The prophet also forbade his son-in-law to force Fatima into polygyny.
4. Polygyny is the source of massive spread of STD:s around the world. Everything from papilloma to HIV to gonorrhea and yeast infections come as gifts from one co-wife to the others.
5. Polygyny forces families to live in financial misery with men being unable to provide for their families, thus forcing women to be the breadwinners so the husband can spend his time fucking other women instead of protecting, and providing for, his family.
6. Polygyny turns people into liars. Women lie to survive, tell themselves that polygyny is a test, that it is sunnah, that it makes them grow in their deen, that they are the favourite, that it gives them the same experience of love, partnership and protection that monogamy offers just not all the time (which is a heinous lie!!!!!), men lie to keep their wives from falling to pieces, they lie about their other lives with their other families, they lie about religion. Polygyny turns all people involved into liars.
7. Polygyny is false, a criminal false interpretation of religion that turns men into animals, women into ghosts and children into semi-orphans.
Please, feel free to add more reasons should you wish!