Times were different and the Kennedy men had a different view of sex.
Joe Kennedy taught his sons to enjoy sex period, without much regard to 'fidelity'.
He married a devout catholic woman and she was expected to turn a blind eye to his mistresses while raising the family. His sons were taught accordingly.
There have been recent commentaries noting the high correlation between powerful men and adultery/sexual promiscuity.
It wasn't 'rape', any more than a rock star who has sex with groupies and tosses them aside is 'rape'.
It is viewed as despicable by many - it's not very 'respectful of women', but was consensual - and I suspect more objections by men are jealousy than morality.
I'd say it was an 'entitlement' feeling, just as movie stars - who JFK enjoyed spending time with - have the same issue commonly.
He expected Jackie to be a good mother and turn a blind eye to his adultery; her vacation with Aristotle Annassis on his yacht while First Lady, who she later married, says something.
He was taught that a lot of sex was good for him, and it may well be the case. Who doesn't 'feel like a million bucks' with that flattery and pleasure?
It raises questions about the role of sex and power. Some men don't - Carter and Bush, for example - while others do, Clinton/Eisenhower/FDR for example.