If memory serves, the first hint of extramarital affairs was in the book "The Death of a President" in 1967.
en.m.wikipedia.org
I must couch this in a context. I say the following as absolutely no excuse for his behavior, but as a comment on the social understanding and narrative of the Kennedy affairs as a concept:
In the 1960s and later, politician's private lives were not as looked into as today. You would certainly have what would make for good PR and photo ops. But it was very much an official version. It was the culture and if nothing else, it was for press access. A journalist may know something as an open secret that the average American will never know. That is not necessarily an era of lies. But it is an era of truth until we just don't talk about something. That is a lie of omission but not a lie of distortion (albeit a lie of omission can create a distortion).
Our understanding of the Kennedy affairs as a meme (or this idea that has a life, personality, mood and narrative in the abstract that is part of the collective unconscious) is heavily influenced by the details of actual history. It was a society after his assassination, after Vietnam where the individual understanding of the world was only as expansive as three television channels, as much college education as the individual may have, books written in a flat, surface level style (comparatively), newspapers and whatever lived experience they had. It was a lofi access to the wider world, and based on the information presented by those areas and shaped by narratives of those areas.
This relates back to distortion by omission. Kennedy was the big infidelity story that leaked early on in this modern history we've lived in since 1963. It looked like Kennedy was unique in that context of normal being fidelity and here is major infidelity. But that context was wrong. The unfortunate reality is that politicians and men with power in that era were frequently promiscuous. Lyndon Johnson had many affairs, as did Nelson Rockefeller. An investigation into an call girl service with connections to East German spies was shut down because it would have brought down half of Washington. Hoover had a dirty log of blackmail precisely because of these infidelities.
The summary of all that is: Kennedy was not unique. He appears unique because the information broke and understanding of that information came in an era that did not know their leaders were doing bad things. Therefore the narrative, even subconsciously, is JFK as an outlier in adultery and that as a defining character trait. That view remains even despite the academic context broadening with further information (sometimes unverified) of other adultery by other individuals in positions of power.
In a scenario where Kennedy lives: firstly, the context of everything would be different because the mental context of OTL America was defined by an abandoned, bloodstained Lincoln Continental pulled over to the side of Route 66 (if I can get poetic). Secondly, there's no more reason to think they would have become public anymore than Johnson's affairs had in the same time frame.
They may eventually come to light in later decades, possibly during Kennedy's life time or after his death. However, that depends on the evolution of the manners and moods of the press, academia, politics and populace. That is such a cascade effect of different possibilities and interrelationships that I can't fathom it here.
