I am the Kat that Walks By Itself

It's at least 30 years since I read the book, but I remember that my reaction to the ending was much the same as yours. I think it must have been Robert Heinlein's last novel before he went decidedly weird and seemed to become obsessed with sex (though I believe it was not published until a couple of years after "Stranger in a Strange Land", which I categorise as his first "weird" novel).