I don't believe that. We have more control over what we think and feel than we are brave enough to admit. The same argument you are making about fat has been made about races. Take people of Asian descent.
The Yellow Peril was a belief about Asians that held sway in this country for nearly a century and a half. Big teeth, slits for eyes, and all the men had an encompassing desire to ravage white women and the women were either docile slaves or wanton harlots, not to mention all Asians were dim witted and scheming and drug addicted. Now these same people are the model minority we should all aspire to be. ideas changed. Now the model minority stereotype has problems of its own (just read Asian-American blogs and the people themselves will tell you what they are) but this a evidence things can change.
Disease is no longer believed to be demon possession, cancer is no longer a thing so fearful it can only be spoken of in hushed tones, and as is proven every July at my family reunion, all black people DO NOT have rhythm.
Now you don't have to find heavy people sexually attractive. We all have our type, and we all have our kink. But it is well within the capabilities of human beings to not treat fat people like something you need to scrape off your shoe before going into the house, and to make treating fat people like skinny people with regards to everyday interactions into a social norm.
Again, you're talking about two different things.
Let's scrap weight and talk about something more fundamental to attraction: symmetry.
People who are more symmetrical are scientifically more attractive than people who are not. I guess the underlying evolutionary logic would be that if your genes want to propagate themselves forever, you want to reproduce with someone who is going to produce more attractive kids, so you are attracted to people who are biologically attractive. Does that mean that every person everywhere is always going to be only attracted to perfect people? No. Does that mean that people can't date people without perfect symmetry? No. But few things in life ever work in absolutes. It's a probability thing: people are more likely than not to date symmetrical people.
Now take that concept and revert it back to weight. There was a time when weight was a sign of fitness because it meant that you were prosperous enough to actually have food to eat. Therefore attractive. Nowadays, greater weight is actually just more likely to point to a lack of fitness in some way.
Overweight people are literally less sexually fit than people of a healthy weight, and so in probabilistic terms people are more likely to not date them. Sure, that sounds cruel as fuck, but that's the biology of it, which is what
DrZoidberg is referring to. It doesn't mean you'll never have a parter, relationship, or get laid, it just means that you are statistically less likely to do so, because you do not meet the ideal image of genetic health.
Now, what you are talking about is how people actually
value overweight people. This isn't a problem that's specific to overweight people, it's a problem specific to people being complete fucking ass holes. Everyone, everywhere who doesn't fit some norm in some way, is probably going to be outcast by someone else, somewhere.