I have relatively small breasts, and I wear a bra when doing anything strenuous - like very brisk walking, exercising (mostly the aforementioned very brisk walking only!) because I DO feel they hurt if they flap around too much!
However if I'm having a sort of lazy day, either just in the house, or just going to local shops, school run etc, then I don't bother because I feel more comfy bra-less when not doing much physical activity likely to cause flapping about and pain!
I couldn't care less what people think, or if my nipples are sticking out etc.
I don't think a well-fitting comfortable bra is necessarily an anti-feminist thing - I do think however that the really ill-fitting, itchy-lacy, and 'gel-filled, push them up to your neck' bras look like they would be agony to wear and probably only designed to please men.
Also, I can't see how not wearing a bra would make boobs sag. As far as I know, it's all about genetics, age, pregnancy, and breastfeeding - and sagging is NORMAL anyway - I wish people would get this. There was a documentary a couple of years ago about African Tribal People. My partner commented that they seem so much happier than us about their bodies and the role of their bodies not just being sexual. The women go topless all the time and make no efforts to hoist anything up at all. They just 'let them hang out' (as he put it) and feed babies etc. They didn't seem to care or 'feel less of a woman' (as I think women in our society do when naturally their breasts sag) if their breasts were deemed as 'saggy' or 'ugly' etc = simply because it never seemed to factor in their lives that one shape/size of breasts was favourable over another. Brilliant.