To be fair, I must note that, yes, of course there is a natural, physiological element to this that is not 'purely' socially motivated; What you're getting at, I think, Count (Though... I don't see any mention of turnips there, so it might be an imposter...), is that in humans, our breasts absolutely are a secondary characteristic; unlike a lot of other mammals, we retain our breasts even when we're not nursing, because they are a part of our 'display', as it were - a feature that says to the animal brain that this creature will be able to rear offspring well. Males have similar secondary attributes that advertise in similar ways - I mentioned a few earlier - that say that they are are fit, healthy and a good choice. In no case are any of those features inherently sexual, on their own, and that's the important point.
The social element is important here; we are at our cores, social creatures, and over time as a species we have developed a wide spectrum of social cues that functionally negate, or at least counterbalance, the individual creature element. We choose partners on a far, far broader spectrum now, to the extent that our choices are not even reproductively motivated most of the time. The social element then, is the acknowledgement that while it is entirely natural to notice and even appreciate secondary characteristics for the way they make our brains happy when we do, that does not, and should not define or control any of our social behaviour or how we treat one another when discussion of direct attraction is not on the table... this along with the social awareness not to *put* it on the table when it is not appropriate to do so.
In most cultures where covering the breasts is not an expectation in various public situations, you'll often find that the same is true or the actual genitals as well; they may be covered by functional clothing, and they may not be, but the main observation is that it doesn't matter, because the day to day situations are not sexual, are not viewed as inherently sexual, and... to be blunt... they're bits everyone has in some form or another, and they have very little meaning outside of actual sexual circumstances. I'm not from any of those cultures, and casual at-home nudism really doesn't count... but I do feel fairly safe in asserting that even in those places and amongst those people, yes, folks of all stripes enjoy those features, just as anyone else might. In the case of breasts in particular, yes, for many women they are erogenous, and so naturally they will have a part to play in sexual situations when those circumstances do arise... but dressing down in the evening over camp duties amongst friends and other non-intimate companions is not such a circumstance, so whether someone is wearing a shirt or not should not matter, regardless of what's beneath.
And besides... Halsin has bigger turnips than me anyway, so the imbalance is double silly ^.~
