In warm months it's easy to keep a bag dry. You night get some condensation in your tent/bivy that then drips onto the outside of your bag, but the down itself will stay dry. The danger comes when it's cold, then your sweat can actually condense inside the down. This is why some people recommend using a vapor barrier inside the bag to stop water evaporating form your body ever getting to the bag. You'll stay warm, but it sounds very uncomforatble to me.
is this based on theory or experience? I used a down bag for 100+ days at sub 40F temps and I never had these issues.