Yeah, Hoover spent a lot of time (and money) trying to get the goods on the Kennedys. Using the FBI to gather dirt for blackmail wasn't invented by Nixon. It's what gave Hoover his power.
Somewhere in the still-locked JFK files is the testimony of those "experts" that Hoover brought in to identify the man in the tape. I imagine that would make for some hilarious Comedy Channel material.
The only reason why I think there's a remote possibility that the man in the tape could be JFK or RFK is that filming yourself in the act with Marilyn Monroe would be the ultimate in ego trips.
It would be a dangerous indulgence in highly risky personal behavior. If nothing else, that's a Kennedy family trademark.