There was also a difference in the 50s where people really were not expected to be at all monogamous until they were engaged. It was normal to have several romantic relationships going fairly seriously at the same time while you decided if you wanted to marry any of them, and it's kind of weird how we sort of silently lost that over the last couple of generations, to where now there's a norm of serial monogamy through your entire dating life. Which doesn't make the possessiveness any better, but it does read a little different in the context of "you need to *actually* break up with your other partners when you get engaged" than in a context where they are clearly already exes.