Funny how so many generalizations are made when it comes to obesity and weight. BMI is a decent guideline, but if you are unusually muscular you'll come out as overweight even if you have <10% body fat. And as has been noted, there are plenty of people who are overweight for reasons other than simply eating too much.
The other interesting thing about obesity is that it is its own punishment. Other than being discriminated against, there's no real negatives to being black, or gay, or a woman, or jewish. But being obese gives you a slew of other health problems. Why pile on a negative stigma on top of that?
I don't mock obese people or overtly discriminate against them, so far as I am aware. But I don't date women who are very overweight. I've dated women who were somewhat overweight, say 20-30 lbs, but nonetheless attractive. And as I think about this, I've dated crazy women, women with serious medical conditions, amputees, women of various ethnic groups, religious women, women with kids, divorced women, and none of these things were deal breakers from the start like obesity is. If there are others with similar experiences, this may mean that in a way, obesity is the worst stigma out there.