Let them call it Fratrimony for lack of a better word. And a man cannot have a husband, that's just creepy. Call the other guy a brubond.
hus·band /ˈhʌzbənd/ Show Spelled[huhz-buhnd] Show IPA noun 1. a married man, esp. when considered in relation to his wife. 2. British . a manager. 3. Archaic . a prudent or frugal manager. verb (used with object) 4. to manage, esp. with prudent economy. 5. to use frugally; conserve: to husband one's resources. 6. Archaic . a. to be or become a husband to; marry. b. to find a husband for. c. to till; cultivate. Use husband in a Sentence See images of husband Search husband on the Web Origin: bef. 1000; ME husband ( e ), OE hūsbonda master of the house < ON hūsbōndi, equiv. to hūs house + bōndi ( bō-, var. of bū- dwell ( see boor) + -nd prp. suffix + -i inflectional ending) Related forms hus·band·er, noun hus·band·less, adjective un·hus·band·ed, adjective Synonyms 5. preserve, save, store, hoard.
You can already leave your property to anyone you wish. There are wills and power of attorney. To call that anything recognizes a relationship and that will end up being the camels nose in the tent.