There is a possibility that Marilyn Monroe and John F Kennedy had one intimate encounter, but they most certainly did not have any kind of affair, seeing as they only met a handful of times at mostly public events. However, because of Robert Slatzer, who kept selling ever more lurid tales to the tabloids, the myth of the actress and the President has snowballed out of control and has been repeated so often that the general public just accepts it as fact even though it's fabricated.
The image of Marilyn that is projected through this tabloid lens is not that Marilyn that her fans know and admire. She is portrayed as a a needy, clingy, pathetic, vindictive woman who was utterly delusional....and that's not who she was at all. She was strong and tough. She was a woman who walked out on one marriage that didn't support her career. She was a woman who left someone she loved because he was abusive. And yet people think that she was so delusionally love struck with someone she met three or four times that she was going to throw away the career she worked so hard for, that she had just undertaken her own publicity blitz to restore her standing with the public, that she would throw it all away being a catty, classless, vengeful mess of a woman? That a woman who said that the thing in her life that she was most proud of was that she had never been a kept woman, the woman who was always classy and discreet on the few occasions she spoke of her past relationships, suddenly had a total personality shift and became a hysterical pathetic overwrought person who would destroy her own career and everything she worked for?
Do your research. Don't blindly believe what a bunch of people who had ulterior motives (money and fame for themselves) say. The people who made these claims, every time they stopped getting attention, *suddenly* remembered something else they forgot to mention previously....oh, I went to nude beaches with her and JFK! Oh, I was the last one to speak to her that night! Oh, I was secretly married to her!
Look for proof. Learn who in Marilyn's life was a credible source and who was not. Don't repeat something as if it's true because you maybe sort of heard it somewhere one time. Get comfortable with the idea that something you were certain was true just might not be.
Marilyn Monroe and John F Kennedy both accomplished a great deal in their short lifetimes. Both should be remembered for those achievements, not salacious fiction peddled in the tabloids.