Read something by the Marquis de Sade. He was big on bewbs too, and that was long before printing anything but basic woodcuts was possible. Men like boobs because we liked boobs as babies. Not much really changed since.
I would say that it probably started with Adam and Eve. When Eve took a bite of the apple. Well, I am sure you know the story. I feel that breasts were God's gift to women, so they could feed their babies. And still to this day, it a good thing for babies to have breast milk.