meelsie, I think the bottom line is that almost any food has the capability to contribute to a person gaining weight. In this sense, theoretically you could eat so many beans that it could cause you to gain weight.
Cyndi noted that she considers 1/2 cup a serving and she has 3 servings per day. murphmitch mentioned that she has no problem eating a can of beans with salsa FOR dinner. Since Cyndi is a vegetarian, she isn't eating higher calorie meats with her beans, and I got the feeling that the can of beans w/salsa was Annes dinner. In both situations, it seems that the beans were actually a low calorie option for a meal.
For me, I am consciously adding more beans AND less meat. I'm actually exchanging a lower fat/calorie protein for the higher one. So I guess you have to look at the rest of the foods you are eating in a day and try and figure if the quantity of beans you are consuming fit into your calorie range. Now I know that counting calories isn't a requirement of SBD, and I don't count, but I am very aware that calories matter.
Last night I had a meat substitute/bean combo with a mexican dinner. I know that my meal was a better choice for losing because my fat and calories were substantially less than if I would have had ground beef/beans.
I hope this helps.