In the many, many cultures (all in hot areas) where female toplessness is/was traditional, female breasts are considered not as to barely-even sexual (and whatever body parts are traditionally covered, usually the buttocks, thighs and genitals, in particular are regarded as very sexual). I’ve read about it quite a bit, it seems to be a very clear and observable phenomena.
Also, anectode, but I’ve heard a lot of guys who traveled or stayed in the more liberal areas of Europe talk about (or lament) how relatively desensitized to the sight of naked boobs they became.