It started with Antonio Gramsci in the 1920s. It was imported to the US with the Frankfurt School of German Marxists in the 1930s, who later brought us a transformation in psychotherapy (Maslow), education (Marcuse), and feminism (Fromm).
The destruction of the American family was done deliberately, because it leaves the almighty state to raise the children, by default.
LBJ made it government policy and this is when it became widespread. Earlier it was shameful to have an out of wedlock child not a career. LBJ provided the means for this to fuel itself into the whirlwind of bastardy we have today.