There were plenty of stupid extrapolations in the discussions about gay marriage over the past couple decades.
I want to marry my mom! (Oedipus !)
I want to marry my self! ( some joint-return tax advantage being the biggest real deal there)
I want to marry all of my teammates !
and… well what about that polygamy thing that we outlawed way back when. (why just one?)
I want to marry my corporate person Acme enterprises ! (if we constrain to Supreme-Court-style persons)
I want to marry my cat/dog/gerbil !
I want to marry my lawn !
I want to marry my cannabis plant !
Where some of these fall apart clearly is one of the parties’ inability to consent.
Where consent is possible, I’d say you can write a contract that will fulfill most of the practical aspects you might theoretically get out of a marriage.