I feel like an actual discussion of this would be interesting to take part in...if this was another forum. But because it's Anime Vice, that won't be happening.
It's sexualized in at least American society, and it's known as a erogenous zone, but compared to other erogenous zones on a female (like a stomach or legs or whatever), it's taboo to show bare female breasts.
Also, they're distinctly a female characteristic. Women have breasts, that's what makes them women (or at least that's how we know them as such), so it's tied to their sexual nature, and our sexual nature as well.