So what is the deal with when people say "you're not attracted to her for HER, but just because you're attracted to fat girls"?

What I mean is, I can like a woman who is chubby because I like chubby women, AND like her for her. I can like her for both. I don't see why people have to say "you just like her because you like fat women", as if there is a difference between the two. It's like saying "oh, when you picked your car, you just picked a car, but you didn't like the car you picked, you just liked cars". It doesn't make any sense.