A glimpse of cleavage is more enticing than the boobs dropped out of the bra. This is what more exciting about the semi naked position. You feel more inclined towards the body when you see a part of it.
Well this might seem strange, but I actually find semi-naked to be more exciting. When a person is fully naked it's like there are no secrets left, everything has been revealed. Being semi-naked however, leaves room for the imagination..
