I understand the reaction completely and feel it is completely normal and expected.
We thought he was 'one of us' (straight, all-american boy, married and dedicated to the same woman for 50 years, honest and strong, testosterone male) and Brando claims he was not like that at all ( sexual, secretive, infidelity, deceptive) etc
We like people who personify, become the embodiment of what we believe in (straight, etc) and we admire them when they become successful while doing it.
Having someone spread allegations that he wasn't like that at all (straight, etc) is not what we know from him, now is it?
Disappointed? If it's true, then of course.
I still think it's all bull .
Marlon Brando is just speculating. "He didn't fool me" was what Brando said...
Newman was married, happily for a long time. I doubt any of this is true especially since he's dead now.