I don't believe there is such a thing as real bisexuality in a man. Some gay men can function sexually with a woman and might even enjoy it. More importantly they might feel a real emotional connection to a particular woman and even be their life partner. Charles Laughton and Elsa Lanchester, Montgomery Clift and Libby Holman, Johnnie Ray and Dorothy Kilgallen, Lou Reed and Laurie Anderson. But these men were/are gay in a strictly sexual sense. If you pressed them on it, they prefer men sexually. Why go through the all hassle of being gay unless that was your real preference.
Maybe its just semantics but I just don't believe there is an equal preference for both sexes. A straight man is never going to want to be with another guy. As for this fad of practicing oral sex with whoever, I would say any "straight" man who really enjoys sucking dick is probably a closeted (to himself) gay guy. "Not that there's anything wrong with that!"
As for women, I think this business of natural bisexuality in women is overrated. But like Freud, I don't pretend to understand women's sexual psychology. But I suspect the rule applies. "Bisexual" women actually prefer other women but can enjoy an occasional man once in a while. If she's attractive enough, she will certainly have the opportunity to sleep with men even if its not really her thing. But straight women who really like men aren't equaly attracted to other women though they might appreciate their beauty.
I knew a provider well who said she was bisexual but I know she never felt the same attraction for men that she did for women. She would really yearn for a woman even though intellectually she got along with men, especially gay men, especially drag queens and transsexuals, who are fake women!
So bisexuality really means gay men and women who are attractive and open-minded enough to fool around with the opposite sex whenever the fancy strikes them