As with so many things, when it comes to virtue signaling, and anything related to the social justice ideology, "fat acceptance" might have had a somewhat reasonable point, to one degree or another, but it has evolved into a completely ridiculous denial of reality. And, likewise as often happens with these things, the original idea is being used to justify the ridiculous extreme. In fact, they are even calling it "fat pride" now, as if there was something prideful about being obese.
The original idea stems from the desire to end "fat shaming" in our society. Kids making fun of, or even harassing, obese kids. Adults making nasty remarks and having all kinds of prejudices about obese people. And so on.
Fine. Some degree of decency and good manners could be expected of society. We shouldn't be making fun of, or shun, or harass, anybody because of how they look, or any other sort of external characteristics. If they have some kind of personal problem (such as a health problem) that may or may not be solvable, that's one thing, and possibly worth of serious discussion, but treating them badly just because of some physical characteristic is not acceptable. As long as they aren't hurting anybody else, we shouldn't be attacking them in any way. I can get behind that.
But the "fact acceptance" movement has gone well beyond that, to absolute ridiculous extremes. It has transformed from "I know I have an obesity problem, please stop making fun of me, it's very hurtful and it isn't helping" to, effectively, considering obesity to be an innate characteristic of the person. In other words, in the same way as your skin color may be innate, or eye color, or hair color, or the size of your ears, or the shape of your nose, or your height, or the size of your head, these "fat acceptance" advocates seem to be promoting the idea that weight is a completely equivalent innate characteristic. In other words, in the same way as you can't help your height, or the length of your arms, likewise you can't help your weight. They don't even use the word "weight"; they use the term "body shape", as if it were indeed an innate characteristic.
Moreover, and more dangerously, many of them are advocating for the concept that people can be healthy at any weight. Here is where they are going full reality denialist. I'm not even kidding; some of them really and truly are promoting the idea that morbid obesity isn't unhealthy.
Of course we know as a scientific fact that morbid obesity increases the risk of all kinds of diseases, many of them by several orders of magnitude. For example the risk of diabetes is enormously increased; the more obese the person is, the higher the risk is. (Some studies estimate that people who are medically considered obese are 80 times more likely to develop type 2 diabetes than people who are not medically overweight. The risk only becomes higher the more overweight the person is.) And diabetes is not just a minor nuisance; it's a rather serious disease.
And that's only one of the common obesity-related diseases. The list of them is quite large. By the age of 40 or 50, the vast majority of morbidly obese people are on one or more medications for conditions directly caused by their obesity.
Some more cynical people say "they are not healthy; they are dying". That's actually not very far from the truth. Morbidly obese people do have significantly lower life expectancy (sometimes by as much as 20 or even 30 years).
And the thing is, obesity is a choice. It's not an innate physical characteristic. It's not a congenital condition. It's not something that you can't help. It's a life choice. And more importantly, it's not irreversible. No matter how morbidly obese you are, it's still not irreversible (even though to such a person it might feel so). Many life choices do cause irreversible, or only partially reversible damage, but obesity is not one of those. If you are still young enough to be relatively healthy, it is perfectly possible for a person to lose all of that extra weight, and decrease the risk of all obesity-related diseases significantly. In fact, even if you do already have some obesity-related disease, it's often reversible by losing weight. For example, if you have type 2 diabetes that is caused by obesity, dropping all that weight very often cures the disease.
Of course nobody is saying that losing weight and becoming fit is easy. But it's still a choice. It's not a congenital condition that can't be helped.
The "fat acceptance" movement is dangerous because it's telling obese people that they are fine as they are, and don't need to lose any weight.