Yes: questions like these need to be asked, and at least tentatively answered, before legally sanctioned poly marriage can even be put on the table for discussion. In particular, there needs to be a clear idea of what happens to the children under the various possible scenarios. I don't think just letting people make whatever agreements they want is going to fly, at least not once any children at all are involved. These things could get very
very complicated. There needs to be at least a framework, a sense of what structures
can work, as borne out by experience.
I have a suspicion that in the end, it will turn out to be best not to make any special legal provision for poly marriages. The law will recognize couples, and those couples will be free, as indeed they are today, to make whatever informal arrangements with other parties they wish. In a few areas -- hospital visitation rights are a good example -- I think it would work to detach the right from legal marriage altogether, and just let people declare who may visit them, etc. But the legal system has to know who is legally responsible for each child, and I don't think it can be more than two people -- even two is often problematic, as we know from long experience.