 According to Gary Westfahl, 

In books written as early as 1956, Heinlein dealt with incest and the sexual nature of children.Many of his books including "Time for the Stars", "Glory Road", "Time Enough for Love", and "The Number of the Beast" dealt explicitly or implicitly with incest, sexual feelings and relations between adults, children, or both.The treatment of these themes include the romantic relationship and eventual marriage, once the girl becomes an adult via time-travel, of a 30-year-old engineer and an 11-year-old girl in "The Door into Summer" or the more overt intra-familial incest in "To Sail Beyond the Sunset" and "Farnham's Freehold".