I am skeptical about this. I understand that in many cultures women didn't cover their breasts. But I would be very surprised if breasts would not still be an important part of sexual foreplay.
I think breasts play a part of determining feminine sexual attractiveness even in cultures where they are not covered. Fertility symbols seem to prominently feature them across cultures.
This explains the evolutionary tendency for much larger breasts than necessary for feeding children. I have read the hypothesis that this change happened when humans started having face to face sex (where the breasts are more accessible) than with the normal male behind female)position you see in other primates.