It's not fatphobic to not be sexually attracted to fat women. Just as it's not brunettephobic or gingerphobic to prefer blondes, or animalphobic to prefer humans.

Besides, it's not a black-and-white choice between fat or skinny. I'm not attracted to skinny women, but I'm also not attracted to fat women. I'm attracted women with a nice shape who are of a healthy weight.

Being overweight is not healthy. And that's not a fat-phobic thing to say. It's just a medical fact.