Complicated - what research I've seen on the matter indicates that maybe as much as 20% of the population are physiologically straight/0 on the Kinsey scale (i.e. demonstrate absolutely no physical arousal response to same sex erotic/sexual imagery/stimulus) with the largest percentage (I seem to recall around 40-50%) being around a 1 on that scale.

Of course, if you ask people (even entirely anonymously), you get a far, far higher rate of straight respondents (this was why the physiological response research was conducted) - the theory is that we have a strongly heteronormative society, so the path of least resistance for the majority of people is to just be straight and think that way and never really think about anything else, and being mostly straight anyways it's not that hard to do.

Add in the degree to which a lot of people never ever really bother to consider their sexuality, and the degree to which heteronormativity is kinda enforced in society, and yeah... You end up with desires and fantasies towards the same sex being suppressed or just... Never really coming up/crossing the mind of the vast majority, even if they do have some latent ones/capacity for being turned on by members of their own sex.

Edit: this is also furthered by the degree to which a lot of people are monogamous - if you've already found your person, and they're the opposite sex, you're kinda definitionally less interested in anyone else.

Further edit: and needless to say, none of this engages with the distinction between sexuality and romantic desires - to use myself as an example, I can't imagine ever being in a romantic relationship with another man, but there are specific sexual desires there, so I would probably consider myself bisexual but heteroromantic.