Reference to hypothesis about breast sexual attractiveness?
Eventually, I found it, here it is:


  In considering the human animal, zoologists proposed that the human
  female is the only primate that possesses permanent, full-form breasts
  when not pregnant. Other mammal females develop full breasts only when
  pregnant. The zoologist Desmond Morris proposed that the rounded shape
  of a woman's breasts evolved as frontal, secondary sex characteristic
  that is a sexual-attraction counterpart to the buttocks, and so
  encouraged frontal copulation, the reason being that while other
  primates mate by means of the rear-entry position, the upright,
  bipedal human being was likelier to successfully copulate face to face
  in the missionary position.


https://www.reddit.com/r/askscience/comments/uflgy/why_are_breasts_so_attractive_after_all_theyre/