That some people do want to be parents?
Someone else mentioned The Unpleasant Profession of Jonathan Hoag, a detective story Heinlein wrote in 1942.
The detective's wife is called out in the story as being the brains of the outfit, a very progressive notion in 1942.
Exceptionally competent women have been a staple in Heinlein stories forever.
I think what people would most likely object to is the implication that every woman should want that.
The closest he gets is the characterization that's it's natural to women to have babies because their bodies are capable of it, and most women do or at least want to. That shouldn't even be controversial because it's borne out by all of human history.
No, its pretty directly in his interview response, not his work: “I don't think there's much future in a gal who's thoroughly opposed to having babies.”