I'm saying it would stay the same (unless you choose to make it otherwise, but that's outside of the discussion)

With regards to if a person would want the first marriage to have benefits more than the others, that's quite an odd thing that couldn't be explained by simply transferring over to polygamy.

A marriage is a marriage, and the benefits are equal to what's currently marriage. If you're arguing for something additional, than that's additional, but simply widening the definition to include polygamy doesn't count that.

The same argument for "marrying for the benefits" can be applied (and was) to same sex couples.

An additional married person would be added to your taxes the same as the first married person was. I could see a slight argument over whether it should increase deductible maximums and what have you, but I don't see why? As long as you don't change anything else, you would get one set of tax laws for single people, one set for married filing jointly and one set for married filing separately. That's how it is in America, and I don't see how that changes anything if it's two people in a marriage or seven.

And with regards to people divorcing, there's nothing special there either... it's completely dependent on how they were married to begin with, and doesn't change anything from the current law besides the number of names on the certificate. Nothing to be figured out.

If they all sign the same certificate, and one wants out, they all go in front of a judge (same as now) and decide how that's done.

If they sign separate certificates, only the ones involved go in front of a judge.

It really all is same as now... just with more names on the certificate (I don't know how the Netherlands deals with it, so maybe it is different for you)