Heinlein wrote about sex the way Robert E. Howard (Conan) did. I.e. he imagined it a hell of a lot more than he did it. Writing about women dropping their clothes and parading about naked is not sex.
Howard died a virgin. Heinlein was married thrice. If it wasn’t for that one scene in Farnam’s Freehold I would have said he never had sex either.
Heinlein kind of, sort of fortuitously tripped over the sexual revolution with Stranger in a Strange Land (only a few years after being accused of being a militarist after Starship Troopers) and clumsily made the most of it.
RAH was a twisted socialist, an EPIC disaster.