Well, evolution as a whole has no purpose, but it does give results that act just as if they had one. There’s a meaningful sense in which a lung is meant for breathing, a wing for flying, or a peacock tail for attraction. The term used is teleonomy; it’s not the same thing as a teleological purpose assigned by intent, but it’s enough of a purpose for informal sense.
That’s kind of a nitpick, and I do agree with the rest of your post - an evolutionary benefit doesn’t determine how we must act, and singling out nipple exposure wouldn’t follow either way. I thought I’d bring it up, though, because this is something you hear a lot:
Breasts in human women are unusual for sometimes storing a lot of fat independent of lactation. That might be adaptive, and a function in sexual attraction is a possible explanation. But you could invent other causes; something related to cryptic estrus, an artifact of having children most of the time, some sort of spandrel, who knows.
And while this is a popular answer, I haven’t seen much to establish it. To start off with, like a lot of evolutionary psychology it seems to be based only on western stereotypes, as if “Married with Children” were our gold standard. Any claim about our evolution needs to look at humanity as a whole; do other cultures all treat breasts in such a sexual way? They definitely don’t all cover them.
Even if you don’t go abroad, though, there is a more serious problem. Male peacocks all have large tails, and you can see the size is a major factor in attracting mates. And there are lots of other animals like that, where flashy colors or pheromones are how reproductive value is assessed.
But people aren’t like that - women don’t all have large breasts. Some people never really store much fat there at all, and yet they find partners and raise families all the same. In fact, look at some most attractive or most desirable or whatever lists, and you’ll see women with smaller breasts aren’t even excluded as lingerie models and other professions based mainly on appearance.
So although there’s supposed to be sexual selection for larger breasts, there’s not even a clear reproductive handicap in not having them. The few studies that have been pointed out to me seem to find that there are different preferences for different people and in different circumstances, which is really what I’d expect. People are really variable.
To sum up: it could be an interesting hypothesis, but it seems to need a lot of work before it is anything else, and right now there isn’t enough to say sexualization of breasts is any different from sexualization of necks or legs or chest hair. Not to say it would justify imposing that judgment on everyone either way.
