I think your standards are a little bit fucked up if you think that qualifies as fat. I also think you need to do some (NSFW) googling if you think that fat is anything close to universally considered to be unattractive.
Nobody's metabolism is the same: not everyone works equally to maintain the same weight (a point that you touched on but seemed to ignore for the purposes of the rest of your post) so it's not fair to universally deride all people for not falling into a certain category of body image.
As well, the notion that societal pressure does not cause some women to make unhealthy choices is ludicrous. If you had been merely talking about a bunch of well-natured adults that had not spent their entire lives exposed to a culture oriented towards telling them how ugly they look (watch any cosmetics ad, ever. Even the ones that tell you how beautiful you are are selling you a product that subsists on your lack of self-esteem), then you might have a point. But the fact is the women of our world have dealt with this more or less from day one, and the corrupting influence of culture is not instantly nullified when they turn eighteen.
But really, the most flawed part of your argument is where you assert that if someone makes a choice, it is instantly open season for you to discriminate. This is, of course, the more reasonable variant of your argument, which is actually closer to "if someone makes a choice -- at least, I think it's a choice, I mean, I don't know their medical history, personal problems, or family background -- then it's instantly open season for me to discriminate".