What do you mean “forced”? Language matters here. They aren’t forced. They agree with the rules and therefore shun you.
You do realize that if given the choice between a physical beating and having your whole family and friends shunning you for the rest of your life, most people would go with the physical beating?
People who are inside the JWs cannot just talk to you anyway. They will get caught sooner are later and will be excommunicated for it.
But they agreed to the rules of the group,
Actually no. Most of them don't agree with all the rules, but submit because of that very threat.
and think polygamy is perfectly fine
That's my point, shunning does much more damage to society than polygamy.