There is also a larger moral dimension. There is a limit to how much of a nation's GDP can be spent on health care because other things must be funded too: education, infrastructure, defense, etc. As obesity-related illnesses now consume more than 50% of all OECD health care expenditures, and as obesity-related illnesses are almost entirely caused by people eating too much and exercising too little (despite all the claims to the contrary about genetics, plastics, and presumably space alien mind control), it's clear that obesity results in nations being forced to spend less elsewhere. So that person slumped on the sofa cramming slop down their throats is directly reducing expenditure on schools, roads, power grids, etc. and consuming health care resources that result in people who fall ill due to non-controllable factors like cancer and injuries would otherwise have been able to utilize.

Unless we argue that one should always stay with one's partner regardless of that partner's behavior, it becomes very difficult to argue that uncontrolled weight gain should be overlooked.