I gotta disagree with you here. I'm a 40D and I'm INCREDIBLY uncomfortable going without a bra. Different people have breasts of varying shapes, sizes, and firmness, and what may be right for you might not work for others. If I got without a bra, I can't do any sort of high impact activities without serious discomfort. When I'm not physically uncomfortable, I'm mentally uncomfortable. It's kind of annoying jiggling all over the place with the slightest movement, and I get enough of that with a bra on. And yeah, I do wear a bra because I do like the way it lifts and shapes my breasts, but I'm still comfortable enough to go braless around the house and in certain outfits. But in most cases when I'm out and about in society, it's just much easier on me to slap on a bra. Really, they arent even that uncomfortable to me.
But on the flipside, you're totally right that it's unfair that women are expected to wear bras. I mean, boobs are literally sacks of fat and other heavy tissues, so why is it so unacceptable for them to sag? It's silly. If you don't want to wear a bra, you should be able to without other people taking offense at it. At the end of the day, the way a person chooses to present themselves should be their decision, and people shouldn't go out of their way to harass you for wearing (or omitting) articles of clothing that make you comfortable.
Within a reasonable limit, of course.