To my understanding, most actors then (and now) were bisexual because they had to be, that is to say, had society been more accepting of a homosexual lifestyle, they would have been able to fully indulge their dispositions, but because society was [and is] what it was, they were obliged to also pursue the social "norms" of a heterosexual lifestyle, get married, have children, etc.
In the case of Brando, it can be argued that he was quite a sexual being, his natural preference being men, and Wally being his life partner and primary source of his emotional attachment.
Many years ago, I knew an old woman, now passed away, who knew personally all those great Hollywood legends. She told my friends and I a story about when she first met Brando (who would go on to become a close friend of hers until his death). She was at a party with a bunch of then up-and-coming Hollywood stars, lots of drinking, smoking and carousing. Brando, having already drunk a lot, had come over to her and began to kiss her--which, even into her 80's she averred was the worst kiss of her life. The next day, Brando was to begin filming for Streetcar Named Desire and he had arranged passes for her and her friends to come to the set. She opted not to go and instead found herself on a plane for Paris where she managed to open one of the primary celebrity hotspots of the era. She said the reason why she didn't go to the set was because she understood in her little make-out session with Brando that he really wasn't into her, or into any women, for that matter, and was doing it because he felt he needed to. She commented on how she observed his interactions with the other guys and that there was something much more intensely erotic and sexual with them than with the women he would try to bed. Some time later, when they had well established their friendship, he made it very clear to her that he had found the love of his life but knew that he could never live that lifestyle openly and maintain his leading-man, sex-symbol status.