I believe Wendell has nailed it. It’s an open secret among Heinlein fans that his marriage to Leslyn was an open one. (For what it’s worth, the divorce is rumored to have had nothing to do with their sex lives, but was largely due to Leslyn’s supposed alcoholism at the time. I report this only as rumor, hence all the weasel words.)
It’s worth noting that despite Ginny’s political conservatism, she was a very open-minded individual towards new ideas and concepts, a Goldwater libertarian conservative of the sort not to be confused with those who have made “conservative” a dirty word to many Dopers today. From this, I think it can be said that it’s not beyond the realms of probability that te third marriage was at least for a time an open one, but my impression is that they were both deeply in love and not particularly interested in such relationships IRL, while being open-minded toward them in later fiction. Much of Heinlein’s reticence about his past was not due to personal embarrassment but rather to the economic and social realities of the time – a leading author of children’s books, which was how he was generally viewed during the 1950s, could not at that time have a past full of radical politics and sexual peccadillos (at least what would be categorized as such by 1950s establishment views).