Ugh. I do believe in fatphobia, because I've experienced it. I've always been bigger, with broad shoulders, fleshy thighs, a lot of muscle mass, and a belly. Even when I was taking intense kickboxing lessons for six hours a week and walking everywhere, I still had a gut and by the BMI was "obese". And yeah, I got bullied for it; I was called "fat whore" for the first time when I was ten. Certain kinds of bodies, especially for women, are considered more acceptable than others, and those standards can be quite unrealistic for a lot of people.
I also believe in HAES...to a point. I've known athletes that were 300, 350 pounds, and no, it wasn't all muscle. And these people still get read as fat slobs even though they are completely healthy and active. BUT anything above that, and you've got a fucking problem. The human body is not meant to go above a certain weight, and if it does, it is completely unrealistic to think that it won't put a strain on the heart, lungs, etc. It is not possible to be 400, 500, 600 pounds and be healthy. It's just not.
And you know what? If you are 600 + pounds and happy about it? Fine. I don't care. It's your body, if you want to destroy it, that's your prerogative. If cramming your mouth with pies and sitting on your ass genuinely makes you feel fulfilled and happy, bully for you.