There is no legitimate argument against fat acceptance that I've encountered thus far. Those who claim to be concerned about fat people's health, well, fat acceptance is not against the healthiness of fat people, why would it be?
Why would we be against our own health or well being, for what possible purpose? It is our bodies and our lives and we are living them. We are not laying down and dying and figuring ways to bring about our own deaths. Take a look at the 'sphere, it's about mental and physical well being, about ideas and thinking, taking back our own minds and bodies from everyone including medical professionals,scientist, the establishment and society.
It's about freedom, fashion, looking and feeling good and better about ourselves, what would lead anyone to believe we are against our own selves?
Those who choose to participate in stigmatization and demoralization are against fat people's health and if you participate, regardless of your rationale, you are against the health of fat people.