it's coercion no matter how you look at it. if my friend wants to get married to a girl i hate and i tell him "if you marry her, im not gonna see you again" that is coercion in your book. but saying "you can't say you won't see him again, because that is coercion"is still coercion. you can't force someone to accept someone else unconditionally. the amish and mormons have every right to turn their back on someone who leaves them.

women would much rather share men than men share women because of the evopsych behind each of the sexes. If 10 women share one wealthy man, they can each father his child and he can (resources willing) provide for them. 10 men sharing one woman is the opposite. she will only father a child with one of them, then nine men are supporting another man's genetic legacy without having one of their own.
women who are more tolerant of the idea of sharing men will not be weeded out of the genetic battleground to the same extent a man who shares his women will be.
Monogamy is more benficial to men because 9 of wives the one wealthy man could have had are being displaced to 9 other men who were before without a mate.Every man down the ladder gets something he wouldn't before, while every woman must trade down after the small pool of dersirebale men are taken.