It seems to me that there are a lot of people telling fat women that they're worthless in the sexual marketplace, and rather few saying anything else.

I know a moderate number of fat women who apparently have happy marriages and (from what they tell me) active sex lives.

Possibly of interest: Fat Sex-- fat women discuss their sex lives, which cover the region of possibility pretty well, including lots of men are attracted, true love, relationships are difficult, and nobody's interested. The only way I can see that those women are different from thin women is that I don't think thin women ever attract men who don't want to be seen with them in public-- and not all fat women run into that particular problem, just some of them.

Roissy just doesn't seem to be living in the same universe I do.

Hmmm, I'm sure I've seen Roissy arguing that unattractive women often can't do much to improve their appearance - that makeup etc is pointless, and that women without (for example) ideal bone stucture are unattractive (see the dating market value test). There is therefore not much room for improvement for many women. Athol Kay, in contrast, seems to think that many currently unattractive women can improve.