It's up to the individual relationship to decide what is acceptable and what isn't.
Not really.
In regards to relationships in a culture there are norms and customs that are considered acceptable by that culture. It is perfectly fair for you to not asked to be harmed (aka prosecuted) for your individual relationship, but to expect society to not look down on your abnormal relationship is pure fantasy.
In western cultures (and many eastern cultures) the emphasis is on children. They get the top priority, their rights and "needs" are paramount.
The emphasis on monogamy is related to this priority of children. It has been proven that children have the greatest chance for success coming from a two parent household. That means that any other configuration has the burden of proof that living in a poly household does not impede the development of children.
The smartest thing the gay marriage movement did is focus on monogamy. A stable gay couple is a viable family unit for raising foster children- certainly more so than an orphanage or single parents.
Until the poly people prove that children are better off in poly relationships, society will continue to put monogamy on a pedestal. Start the long-term studies now and pray they are in your favor.
If you don't have facts to back you up, you are basically asking the world to put your rights as an adult (to have any relationship you want) over the rights of children (that MAY be raised in your household). The modern world will reject that request every time.