The intense excitement of the upper centers of sympathy willy-nilly arouses the lower centers.

It arouses them to activity, even if it denies them any expression or any polarized connection.

Our psyche is so framed that activity aroused on one plane provokes activity on the corresponding plane, automatically.
So the intense _pure_ love-relation between parent and child inevitably arouses the lower centers in the child, the centers of sex.
Now the deeper sensual centers, once aroused, should find response from the sensual body of some other, some friend or lover.

The response is impossible between parent and child.

Myself, I believe that biologically there is radical sex-aversion between parent and child, at the deeper sensual centers.