After reading a number of his adult books as a teen I came to the profound conclusion that he had a thing about incest.

I came across
Father/daughter

Mother/son (involved time travel so that son was older then mother)

Brother/sister
(there was commentary about how good and natural it was for older siblings to educate their younger siblings about sex)

progenitor/opposite sex clones! By this time my main reaction was, so that's why they were opposite sex clones.

This coupled with the all-woman-want-to-have-sex-with-me-and-my-perfect-perfect-genes 'hero' completely drove me away from his adult books.