On the pragmatic side, if they are really going to allow multifarious relationships, I can imagine all manner of lawsuits over inheritance, custody, and communal property rights. If one enters into any one of these, they ought to have a contract looked over by an attorney, because sooner or later there will be a complicated dispute.
Wealthy individuals can ‘marry’ their children to avoid inheritance and estate taxes.
Looks like there is already a big duspute about how many genders exist. I know of only two. How many can be legally proven?