I'm not really seeing where anyone is tearing you apart here, although I can tell you're upset with the advice you're being given.

Figure drawing is a skill that requires lots & lots of practice to master, esp. if you're a professional artist hoping to sell your work. (I do illustration/technical drawing for a living, and take add'l. art classes on the side; figure drawing really is a skillset all its own.) Like others have said, nude modeling is kind of a self-selecting profession, in that usually only classically attractive people with self-confidence gravitate to it. Your BF isn't necessarily drawing slender women because he prefers it; he's drawing those women because they are usually the only models available, so please don't take it personally.

I have many drawings, photos & ceramic pieces that I keep in the closet or in storage because I am so critical of how they turned out. My family & friends have seen them, think they're wonderful, and can't understand why I hide them. But when I look at them, all I notice are the flaws. This is magnified in drawings I've done of loved ones (or self portraits). When I look at them, all I remember is how much better looking people are in real life vs. my poor efforts to capture their likenesses. We are all our own worst critics.

I think you're feeling upset because you want your BF to see you as his 'muse', and have a very romanticized idea of what art is. The actual process of art (esp. figure drawing) is not that sexy. It's the difference between watching a steamy scene in a movie & thinking how hot the chemistry is between the actors...versus the actors who are in full body makeup, working under multiple cameras & hot lights for hours at a time to get the right shots on film.