But I'd suggest that there is something more primal and simple than the obvious "forbidden fruit" stuff. It's a double-edged sword, this thing of celebrity. A very lucrative and hyperactive industry is built around the very goal of bringing scores of attractive women to the attention of men and women alike. Men are told that they want to bang Celebrity X, and women are told that, since men want to bang Celebrity X, women would be wise to follow suit and accessorize in similar fashion.
So when grainy selfies of Celebrity X suddenly surface, yeah, there's gonna be a feeding frenzy, precisely because of the lack of pretense or contrivance in the very candor of the photos. Sex sells, who'da thunk it? Men in particular just want to (to put it as crudely as possible) break a nut, as it were, and new stuff is always welcome. Let's not overthink this. It's not just famous boobs, though there's always a frisson to that.
What qualifies as "sexual" can be interesting. Side-boob, cleavage, most of the breast, all but that magical pink or brown nozzle at the end. The nozzle makes all the difference in the world, aesthetically and legally. It's the difference between a PG and an R rating for a movie, or a magazine being moved from the floor shelf to behind the counter.
But again, that entire industry goes to great lengths to give you as much as possible, holding back that 2% of the boob that caps off said boob. I mean, the publicity machine has put Jennifer Lawrence front and center for a couple years now, in a variety of provocative poses and outfits. The only difference between today and yesterday is that now you know what color her nipples are.
Doesn't make it right, and it certainly doesn't make the image of millions of cybergoons spanking their monkeys any less repulsive. But it's no surprise, either. Part of democratizing all this wondrous technology is that it removes any notion of a centralized controlling authority for this stuff. It's a free-for-all out there, and when fans no longer have to wait for the next carefully crafted press release, things can get kinda weird.