It's not a good idea to pick any garden produce while the plants are wet if you have to touch the leaves, unless you are harvesting the whole plant, like beets, carrots, onion etc.

Is it because it causes rust on the leaves? Sometimes I don't have an option, if the beans are ready and its been raining alot.

It will not cause disease, but if you have a diseased plant it will facilitate the spread to other plants.

Thanks rockguy and farmerdilla.