I've heard it claimed a number of times that sexual attraction to women's breasts (1) isn't universal, and (2) is in fact (relatively) unique to the modern West. Very few things are true cultural universals, so I'm not especially skeptical of (1). But I am a bit skeptical of (2), and I want to know what actual historians think about that claim. I'm no expert, but I'm a fan of ukiyo-e and I've seen a fair number of erotic prints from Edo-era Japan (certainly not a time and place of great Western influence) that definitely seem to be sexually interested in women's breasts. But maybe that's a naive reading, or maybe Japan is just an outlier. Is attraction to breasts actually as uncommon in the historical record as it is sometimes made out to be?
I can't really speak to the whole world, but with my experience studying Greece and Rome, they absolutely viewed breasts as sexual. Greek erotic poetry went into great detail on how they looked: the size, color, firmness, etc.
When Rufinus was describing the ideal female body, the breasts were those of Aphrodite (while the head, eyes, etc. were attributed to less attractive goddesses). This likely signifies a belief (at least in his case) that it was the most beautiful part of a woman.
There's also the old legend about the Trojan war that Menelaus was hunting down Helen to kill her for infidelity until she exposed her breasts, causing him to throw aside his sword and take her back as his wife. For unknown reasons, she's known as "the face that launched a thousand ships" rather than "the breasts that dropped a single sword". That seems like relatively strong cultural evidence that attraction to breasts was seen as normal.
Since that only covers the male gaze, there's also a line from one of Sappho's poems:
Where it is allowed to make this thing stand up erect, to grab the breast and touch with both hands
So we can conclude that attraction to breasts was not just limited to men.
Finally, in Roman times, in the Ars Amatoria (a handbook for both men and women on how to court and have sex with each other), Ovid lists off a number of unattractive traits and ways to fix them. One of these is "Is your trouble a flat bust? Wear a good bra*." Given that this was a widely popular book intended to capture the popular beauty views of the day, we can conclude that larger breasts were linked to being more attractive. Ovid goes on to discuss various sexual positions based on the most attractive part of the body:
She who has youthful thighs, and faultless breasts, the man might stand, she spread, with her body downwards.
Ars Amatoria also describes how Paris "was taken to her [Helen's] warm breast" in order to show how they committed adultery, connecting it once again to sex.
There's also just the fact that almost any erotic artwork in Greece or Rome which involved a woman displayed her breasts prominently. Some examples include
^(*This is the Peter Green translation, which takes some minor liberties. Romans didn't have what we'd consider a bra, but they had similar breast altering clothing, so he just translated it as the most recognizable word.)
Just to add to the excellent response of u/EquivalentInflation – a while ago I answered a questioner who asked Do we know what Joan of Arc looked like? My response pointed out that, while we have almost no idea about any aspect of Joan's appearance other than her haircut and voice, her male companions did make some fairly detailed comments on "the pleasing size and shape of her breasts." So we can add that such attraction is not simply "modern" in the west.