My girlfriend almost always leaves her bra on when we're having sex, and if she doesn't, it can actually be painful to her. She pointed out to me that track running women usually have the smallest boobs because it's naturally harder to run with C cup or bigger sized breasts, and that it can actually be a painful experience, even with a sports bra. I'm wondering if other large breasted women have this problem also (and feel the need to support their breasts when going at it)