Beans are Bad for a few reasons:
They are acidic and throw off the acid / base balance.
Their lectins promote gut permeability.
Their lectins promote bad gut flora.
Scary article from Loren Cordain on Legume lectins:
Because dietary lectins are able to cross the gastrointestinal barrier rapidly and enter the circulation intact (Pusztai et al. 1989), they may also be able to interact directly with synovial tissues. Although not a characteristic model of RA with all of its symptoms, a rabbit model of arthritis has shown that the direct injection of legume-derived dietary lectins into the knee joint induces the development of severe arthritis. Specifically, single injections of Lens culinaris lectin (derived from lentils), Pisum sativum lectin (derived from peas), or the lectin concanavalin A derived from the jack bean (Canavalia ensiformis) were able to induce severe arthritis characterized by an amplification of the initial inflammatory response due to T-lymphocyte stimulation (Brauer et al. 1985). Source: "Modulation of immune function by dietary lectins in rheumatoid arthritis,"