BMI => Do you really look good if your mommy tells you so?

In one of my classes recently, we were talking about beauty: various definitions, personal experiences and so on. They argued that Marilyn Monroe is more beautiful than Twiggy because she's curvier (we compared two pictures, one of MM in the Seven Year Itch and one of Twiggy in a tube dress). However, most of the girls in the class are careful about what they eat. Is it unhealthy to be careful of what you eat to remain thin? We discussed the importance of the BMI - here's a table.

Height Weight Range BMI Weight Status
5’ 9” 124 lbs or less Below 18.5 Underweight
125 lbs to 168 lbs 18.5 to 24.9 Normal
169 lbs to 202 lbs 25.0 to 29.9 Overweight
203 lbs or more 30 or higher Obese

I do not remember a time in my life that I haven't been overweight. And I think these girls have never actually been overweight, yet they say that they're unhappy with their bodies and the way they look. I prefer the Marilyn Monroe look myself, but I think it takes either a very confident woman, or a naturally slender/curvy woman who doesn't watch her weight to be satisfied with that look in today's skinny obsessed world. One thing I do NOT agree with is the assumption that it is not possible to change one's weight. I may have a hard time doing so myself, but that's because I eat things I ought not to, and I don't exercise like I should, not because I have some sort of physical deficiency. People who say that it's impossible to lose weight simply haven't tackled their weight problem the right way for them to lose the weight.