Not sure I agree with that. There is no tangible reason why marriage needs to be between two people only, and I'm not sure how it violates the dignity of women. I would argue that pornography does that, or even modelling, yet many people would say the opposite.
Because, in most cases, polygamy is a male only preserve and comes along with a system that gives more power in relationships to men. Islam isn't fair in its rules around polygamy as it does not allow a woman to have multiple husbands. Polygamy is just another part of an entire system that reduces the rights and freedoms of women.
I think the only real way that polyamorous relationships will work is in a system where it is applied equally to both genders and in a culture where it is used by both genders.
In relation to Islam, I think societies are less accepting of polygyny than they were previously, but it obviously does happen and where everyone involved is 'happy' and it works then there isn't much to say against it.
In much the same way that women are happy to wear a hijab/niqab etc. It is a cultural pressure to conform with strict sanctions if they do not (legal or cultural).
We have similar cultural pressures in the UK to certain degrees, girls steered towards dolls and away from construction toys for example. Ask most 9 year old girls and they will tell you they are perfectly happy with playing with dolls rather than boys toys because they have been conditioned to think that way.