I think there are two different things here, though they overlap. All things being equal, people at a healthy weight for their body are more likely to be fit, healthy etc. So for the opposite sex to find healthy weight bodies more attractive than very over or underweight bodies, is a matter of 'natural selection' if you like - we unconsciously choose someone who looks like they'd make a good mate (even if we are only out for the night!).
Of course this is affected by the media, so that we look for 'on the thin side of a healthy weight' because of that, but even so, that's what most of us find attractive.
I don't think there's much point arguing about that. Of course our personalities are just the same - but sexual attraction is based on physical attributes first, the rest comes later. And its basically sexual attraction that helps a girl get a cab etc!
The more serious one is when we hire people for jobs, speak to people in the street and so on. Because that should very definitely be about who they are as a person - so discriminating against fat folk there really is a nasty prejudice.
Our whole society is so image conscious though, that this needs challenging, in all of us, all the time. Are we nicer to pretty people than plain ones, tall men rather than short men and so on.