Definition of fatphobia:

irrational fear of, aversion to, or discrimination against obesity or people with obesity

https://www.collinsdictionary.com/submission/21048/fatphobia

Not being sexually attracted to obese people doesn't mean I have an irrational fear of them or discriminate against them. I'm not attracted to men either, but that doesn't make me homophobic. I'm not sexually attracted to cows, but I don't have an irrational fear of cows.