Bell peppers aren't bitter at all (except for the green variety, which are just the unripe peppers). I find that yellow, orange and red peppers are actually a bit sweet raw, especially the smaller cultivar you can occasionally find. The sweetness after sauteing comes for the most part from the Maillard reaction (which is also the primary cause of the sweetness in caramelized onions and not the chemical process of caramelization, despite the name).