It started long before that. When they got us to accept pre-marital sex, cohabitation, adultery, abortion, etc, etc.
If you outlaw one, you might as well outlaw them all. Keep sex within the bounds of marriage, where it belongs.
With teen sex legitimized, the question is only how early can they begin?
There are some who teach "sex positive" thinking and believe that young children should not have their sexual persona go dormant (as is generally believed). They believe that they should be encouraged to be sexual beings.
Look at allaboutsex.org for some examples of this left coast line of thought.