MH, you seriously overestimate the innocence of children and purity of ages past.
Humanity really hasn't changed all that much, and back in the days before www, television, porn mags, there were people living in much closer conditions and more squalid conditions. Have you seen the sizes of homes and dwellings from back in the day? Kids saw their parents having sex, kids saw animals having sex.
The prudish notions you are claiming to have been of times gone by are pretty well a Victorian construct (before that, a Puritan construct). Society swings in terms of sexual expression - freedom, repression, freedom, repression. The repression often occurs at the hands of religious zealots, incidentally. Once one form is common-place, value begins to be placed on the opposite. It's less common, and therefore more attractive. So more and more people begin to adopt that at ude as being attractive, and the shift begins anew.