I personally believe most if not all men are bisexual, to some degree or another, no matter how minuscule the degree to which one is might be.
I think most people who choose to identify their sexual orientation as either 'gay' or 'straight' do so based on their predominant attraction to either the same or opposite gender, respectively; but that ultimately, there is attraction to both genders in all of us (or at the very least, the possibility of attraction to both genders, whether we choose to acknowledge that or not).
How one chooses to express, if indeed at all, their bisexuality is also extremely variable, and many people (if not most) probably don't realise they are bisexual since the way in which it is (or is not) expressed doesn't seem bi to them.
Perhaps there genuinely are people who are 100% one way or the other, but my guess is that such people are extremely rare. I think, for most us, our sexual orientations are a sliding scale or continuum (of bisexuality).
In my view, nobody chooses their bisexuality - since that bisexuality is just the natural default setting of human (and, indeed, other animal) sexuality. But what people do choose is whether or not they identify as 'gay' or 'straight'; a choice they make based upon their predominant attraction to whichever gender.