"Metaphysically impossible?" or should I say it was physically impossible for either of them to sign the marriage certificate.
Polygamy might not be the way of the future, but the decriminalization of consensual Polygamy most certainly must be that it can hardly be still in the interest of the government to to stop three or four people from living together in the bonds of matrimony especially after civilization fell at least twice between the 1960s and the 2260s.
The XO got that Tattoo in the episode Tattoo when Him and his pappy went for a spirit walk and found a colony of hybrid sky-people living in the jungles of south America I assumed. It's been a while but I thought that the tattoo was the equivalent of getting a T-Shirt at a rock concert since the tribal people gave it to him. i think he might have given different reasons in another episode because Voyager has amusing crooked continuity.
Seven might want her darling to get a neural implant so that they could "cuberfuck" like she used to with Axum, who she might still boff in her dreams, and Chakotay has his own experience with borg implant group sex that he decided that it wasn't worth complaining about... Not being able to cyber each other might be for her like a regular woman would feel when marrying a bloke with no penis, which happens all the time. Although Chuckles does have that strap-on he used in Scorpion which follows the line of thought I was enjoying earlier.