this is where we try to control for the positive material externalities that come from sharing resources among people. you dont have to buy two of everything if you share, it's cheaper to add a room than obtain a whole house and duplicate bathrooms + kitchens etc.
widows would fall afoul of this (in the sense that they've lived with that advantage for most of their lives), as well as obtain a likely wealth effect from inheritance, but it wouldn't surprise me if the entirity or majority of the marriage effect is largely just a restatement of equivalisation + social transfers/safety nets.