My impression of what little Heinlein I read in my youth is that he is advocating a new and fresh view of sex and sexuality. Sex between people of vastly different ages (in "The Cat Who Walked Through Walls" it's a dirty old woman, if memory serves) and the parent-child sex is an exploration of how far people will go with their perceptions of sexuality. It turns out that as long as the fiction is good (and it's very good, imo), they would go pretty far.

As for there being more dirty old men later in his career - well, one can only write about what one knows. And when you're a dirty (at least by society's narrow standards) old man, you'd do better not to try and animate a horde of virile young warriors. It makes for silly heroes.