Actually, the GP is correct. It was something I was going to make a comment on.
In "Time Enough for Love", Heinlein's main character, Lazarus Long, diligently details the implications of twins' bedroom antics, and the potential for corrupted progeny. The suspect twins are being sold as sexual slaves and promoted as "pure" so that they are to bear children with no defects, thereby making them prime retail cattle. He buys them so as to free them from this life for which they were essentially created.
Not only that, but LL replicated himself in the form of two young girls (identical twins), and, at a point, consummates his fatherly/masturbatory/brotherly relationship with the both of them...frequently. None of this is in explicit detail, of course, but it is definitely mentioned so that there is no question of the act occurring.