I wouldn't expect beans to make you constipated. I eat a lot of legumes and I'll admit to having a lot of gas- that is, flatulence- but that's the extent of my problems. I wouldn't think a half-cup of cooked beans at one meal was excessive either; I often have a bit more than that.
Are your beans cooked enough? White, pinto, red, and kidney beans don't take me more than 60 to 90 minutes to cook, and split peas or lentils take even less time. But garbanzos and soybeans seem to take forever, and beans really aren't easy to digest if they're undercooked. (I've heard that raw kidney beans can actually make one sick but I don't know if this is true.) If you're using canned beans, this should not be a problem, but canned beans are usually unpleasantly salty IMHO.
Speaking of salt.... if you're cooking dry beans and add salt at the beginning of cooking, instead of when they're nearly done, this can prevent them from cooking and softening properly.
"Ground Control to Major Tom.... Ground Control to Major Tom...
Take your protein pills and put your helmet on...."
David Bowie- "Space Oddity"