I can think of four instances right off the top of my head, and it's what keeps Heinlein from being my favourite author of all time. It seriously creeps me out. I don't care if you're a thousand years older than the lady- dude, she's still your mom.
However, it was his breezy, post-50's take on sexuality that got him to the bigs with Stranger in a Strange Land -- in which, if you'll recall, the U.S., at the end of the century (this was set in the 1990's more or less, IIRC), was trying to hold off, with defensive diplomacy, a bunch of lowlife international thug-leaders and murderers-in-uniform who constituted the U.N. Anyone remember that one?
Wow, if he only knew.
In 'Time Enough....' go back to the part where he meets his mother...... also, in the same book when Laz was on the prairie and he speaks of young girls (his daughter) growing up and etc. etc..... his later books definitely bordering on that topic in some fashion or another...it's why I got turned off to them in the end.