I think it's primarily cultural. My friend who lives in rural Mexico says breasts are no big deal there because they are seen nursing babies all the time. Where she lives, ankles are sexy and a bit taboo.

The size or shape of breasts literally indicates nothing about health or the ability to feed a baby, so it's really not that, and so many people in the past 50 years were not breastfed, so I don't think it's that either. I find breasts really attractive, but I've also been to topless-optional and clothing optional-spaces where everyone's breasts are on display, and it's just not inherently sexual - that's an oversexualized female objectifying culture. The obsession with hiding the female nipple is another weird American construct. You can show your entire breast in public, as long as you don't show the nipple.

I love your question about is that really a way to "catch a man"? Had a good chuckle.