Umm, so why would an otherwise disadvantageous feature like enlarged breasts (bad for balance, backs etc..) be selected for to such a degree that it was universal among human females. Other than sexual display you got nothing.
Any display that functions as a valid signal of sexual fitness that can be picked up by evolution will picked up by evolution and used in mate selection. Why would breasts be so different? Do you doubt that women are attracted to deep male voices or larger shoulders and men have evolved to overemphasize these traits as a result?
You might say, "showing breasts isn't biologically determined to convey a sexual invitation." This is no doubt true. Then again so little of human behavior isn't so modulated by culture that anything short of penetration isn't determined by biology to be a sexual invitation with high probability. In our culture It's not at all absurd to think a girl might merely be a friend despite the fact that she bent down, spread her legs and rubbed her ass up against your cock if she was dancing with you at a club. Even more can be done in special contexts like movies or film without conveying an actual sexual invitation. On the other hand showing an uncovered face, in other cultures, can convey sexual invitation.
Sorry, but there just isn't a nice line to be drawn. Nearly everything humans do is partly sexual, even the most innocent play in childhood mimes and explores behavior that will later be used to attract mates and nothing short of sex is necessarily conveys sexual intent.
Ignore the idiots obsessed with the connection to reproduction. It doesn't even make sense to connect what biologically follows from child birth to sexual behavior. Screaming in pain is biologically caused by child birth yet is the farthest thing possible from openly sexual.