I respectfully submit that even WHEN it is affected by genetics, it remains a behavioral issue.
I am an alcoholic of the same Dr. Jeckyl - Mr. Hyde type as my grandfather. I am convinced that genetics play an enormous role in alcoholism, and that alcoholism is more like an allergy than a disease. Yet it remains a behavioral issue. Certain behaviors result in symptoms (starting fights and being arrested, in my case!), and a change in those behaviors results in total elimination of the "symptoms."
If alcoholism is a disease, it is entirely self-inflicted and self-cured, regardless of genetics. That's the hard, cold, ruthless reality. Accepting it is simple and true.
A mammal cannot manufacture fat out of zero input. Perhaps some bodies manufacture certains kinds of food much differently than other bodies, hence tending more toward obesity in one than in another. But I would bet you all the tea in China that if I was able to get one fat person, regardless of genetics, to duplicate my exercise and eating on a daily basis for one year, they would lose fat. It's a mathematical certainty.
It's all behavioral. It's just that individuals each have to tune their behavioral meters to widely varying norms. It's not fair, but nobody said life was fair.