If the President of the USA and Attorney General wanted to keep something secret, it’s easier and safer to hire publicists and pay off journalists to squash rumours. Or just let them be, especially if they are featured in a gossip column. Because after all, that’s all it is, gossip.
From their meetings, Marilyn only met Bobby with others present and she had a one-night stand with JFK (which was a common occurrence for him). Marilyn was not special in any regard to the Kennedys other than having a few mutual friends.
Marilyn didn’t have a diary of secrets (or a diary of any sort) that she was going to expose to the world. Because what would they have told her?
To believe that such important political figures would share such information with someone they met no more than four occasions is ludicrous.
However, JFK did have an affair with Judith Exner (aka Judith Campbell). She had evidence to prove she was with him for approximately 18-months after they met in 1960 thanks to Frank Sinatra introducing them both at one of his shows.
On the 7th March 1960, the night before the New Hampshire primary, Kennedy and Exner made love for the first time in New York City, according to her account. The affair continued into his presidency. Judith also was introduced to Sam Giancana through Frank. She says she became a messenger between JFK and Giancana to help him be elected as president. If anyone was to be “bumped off” for knowing “too much” it would’ve been Judith.