Is it true?
My simple answer: No.
My somewhat more complex and nuanced answer: It depends somewhat on how you define your terms. But if you define your terms in any useful or commonly-understood way… then no.
If you define sexual orientation purely on the basis of physical sexual attraction — and if you define “bisexual” as “having any physical sexual attraction whatsoever to both women and men ever in your life” — then okay, yes, I think most people probably fit that definition. Not all, but most. People on the absolute far ends of the Kinsey scale, people with absolutely zero sexual attraction to the same sex or the opposite sex ever in their lives, do seem to be fairly rare.
But that’s not a very useful definition of “bisexual.”
And I don’t think it’s what most people mean by the word.
And ditto with being straight. A lot of people have passing fantasies or curiosities about same-sex sex. A lot of people experiment with same-sex partners. Some of those experiments aren’t utterly laughable disasters. That doesn’t make these people not straight.
So again, I think most people would agree: Having the occasional passing attraction to, or even experience with, both the opposite sex and the same sex… this does not make you bisexual.
For most people who self-identify as bisexual, what makes us bisexual seems to have more to do with the fact that these attractions and experiences are not trivial. They matter to us. They’ve shaped our lives. They’ve shaped how we see gender. They include relationships and people in our sexual histories that are important to us, and that we don’t want to dismiss. They’re an important part of how we see ourselves, and how we relate to the world. Or any combination of some or all of the above.
And that is most emphatically not true for everyone.
If we define “bisexual” as “anyone who’s had even a trivial passing interest in both women and men ever in their lives”… then what would the words “gay” and “straight” even mean? I guess we could define “gay” and “straight” as “being entirely, 100% sexually and romantically oriented toward the same/opposite sex from birth to death”… but that’s not very useful. It doesn’t tell us a lot about a person and how they live their sexual and romantic life. The difference between someone who’s entirely, 100% oriented toward the same sex (or the opposite sex), and someone who’s 99.5% oriented toward the same sex (or the opposite sex)… for most people, I don’t think that’s a hugely important distinction. So this “anything but 100%” definition of bisexual… it’s not just that it isn’t how most people define the term. It also isn’t very useful.
Finally, and maybe more importantly:
Because we don’t have a universally agreed-upon definition of these terms — gay, straight, lesbian, bisexual? And because for a lot of people, these terms are really personal, and really important?
I think that we need to let people define these terms for themselves.
If a woman had sex with her husband twice a week for ten years before leaving him for another woman, but that sex always felt false and alien and detached… it’s not up to the rest of us to say that those experiences were important, and she’s therefore not “really” a lesbian.
And if a man has put his hand on another man’s cock exactly one time in his life, but that one time was a formative experience that shapes how he sees his own sexuality and male identity and how he sees other men and their erotic possibilities… it’s not up to the rest of us to say that this experience was trivial, and he’s therefore “really” straight.
If anyone of any gender has sexual thoughts or experiences about both women and men… it’s not up to the rest of us to say that they’re “really” bisexual. Or, for that matter, that they aren’t. It’s not up to the rest of us to say whether these thoughts or experiences are trivial or important. It’s up to the person thinking them and having them.
Of course some people are in denial about their sexual orientation. When we’re talking about this subject, it’s important to acknowledge that. And I do think that, if it weren’t for homophobia and biphobia (both external and internal), more people would probably act more bisexually. More people might even self-identify as bisexual. But even with all that being said… everyone is still not “basically bisexual.” This is still subjective terminology, and we still need to let people define it for themselves. And the power to name ourselves is too important for us to take away from each other.
Yes, that makes the language less precise. Language is sometimes imprecise. In fact, language is often imprecise. It’s dependent on context, and it’s dependent on culture, and it can easily be misunderstood, and it can mean subtly or not-so-subtly different things to different people.
Suck it up.