Hey, according to the BMI, I'm not underweight. Well, look at that. I'm at the bottom of the healthy weight, to be sure, but not underweight. Heh.
Anyway, this is an interesting discussion, and I'm not sure which side I could see as being right. Heck, I'm a bit divided. Personally, I see a problem with the prejudice (after noting that I myself am prejudiced in this regard... I think. I'm still not entirely sure my attraction to thinner women is due to a prejudice, or a merely being more interested in those closer to my own size); however, I also see the logic in that there is a reason for certain prejudices, ie. it being potentially unhealthy. Now, if someone is 'overweight' but relatively healthy, good for them, that's fine. But really... I can't see that being the case in the majority of circumstances.
An interesting thing, too, what Ramsus said about the old stone-age hunters being the most fit... I quite agree with that. Actually, that was the basic idea for the Greeks, too. Their ideal male was tanned an working-type strong, from working in the fields all the day. It was also looked well upon because such sorts made better soldiers - and all landowners needed to fight (and a phalanx fought mainly by brute strength and pushing.) As for women, in ancient Greece you can see quite a diversity due to culture. The Athenian type woman would have ideally been paler and rather thin, whilst in Sparta, where their women trained to make themselves fit (so that their children would be strong), the ideal woman was fit and strong. I suppose these things really do come down to cultural differences. And in the end, I think I'll have to side with those saying that they aren't prejudices, per say, but rather perceptions. In regards to the goals of a certain society, a certain woman might be more favourable than another. The Spartans like their women stronger, and the Athenians weaker, as one example. And I guess this is what I see with myself, too - it is a reflection of the thing I value. I prefer women who are skinner, it is true, but also - interestinly enough - not really dressed up, with little makeup, darker haired, and with glasses, likely, and bookish. If someone were to ask me my idea, it would probably be something like that. Why? Because, at least in first impressions, that is usually someone that is more intelligent, which is what I tend to look for. It might be a prejudice in some regard, but it is a useful one for trying to find who would suit one best.
Oh, and in regards to what Lord J said about women catching up height-wise... actually, interestingly enough, 5'8 seemed to me to be a very short height when he mentioned it. You see, I think my twelve year old sister is something like 5'6 already. I bet all three of my sisters are going to be my height (6'0) when they get older. My mother, too, is rather tall, being at least 5'10. So if anything I'm more skewed to towards the taller spectrum of things.
And finally, in defence of my having mistakenly put down 120lb there at the beginning, I honestly don't know what a 120lb, or 140lb, or 160lb 5'8 woman would look like. I'm really, really bad for gauging weight, actually.