I'm going to turn down a girl because she is fat
Is that the only reason why you're turning her down?
I think fat girls are disgusting.
I admit that there are some physical characteristics that I find sexy and some not, however you turning her down because you think that fat girls are disgusting is just a very bad thing to say, and I don't think you should tell that to people. In the future, try to focus on the person's personality and other strengths, instead of focusing on their physical characteristics.
I think one day she will die due to her obesity.
It's alright if you don't want to date her but you have no right to judge her just because she's overweight. Have a conversation with fat girls, you'll understand they are beautiful too. It'll change your perspective.