‘Force’ here is tricky. If two people end up differing vastly in terms of what they want in life as a couple, one will often go along with what they don’t want just to save the relationship.
No one is ‘forcing’ them to do so.
If one partner decides polyamory is the right thing for them, does the right thing and tells their partner, and their partner decides to try polyamory for the sake of saving their partnership, no one is ‘forcing’ them to do so.
I always say that is the worst situation to try polyamory in, but plenty of people have that introduction and it still works for them and they end up discovering that they actually prefer it.
Whose to say?
It’s similar to if one person in a couple changes their mind about having children and wants to have children and perhaps the other person decides to go ahead with it even if they don’t really want to to save the partnership. That happens a lot.
And in my opinion, that’s a worse scenario because it involves the creation of a human life that has no say in the matter. Whereas, two consenting adults deciding how to go about addressing an incompatibility only risks those two consenting adults.
Can someone be ‘forced’ into polyamory through an abusive relationship? Absolutely. And I’ve seen this. But the problem is the abusive relationship, not polyamory itself.
If you’re being ‘forced’ into anything, that’s pretty much a problem inherent with your relationship that has nothing to do with whether it’s monogamous or non-monogamous. And monogamous people can ‘force’ their partner into a lot of harmful things in that regard without coming anywhere near polyamory.