This is complicated, and both perspectives can be true at the same time, probably. The question is complicated by the fact that sexual orientation is an identity, not an independent biological phenomenon or even a platonic ideal.

Talking about it can be confusing for some people, especially people like me who are very polarized in sexual attraction. I've lived 60 years without feeling the faintest sexual attraction toward woman, so I'm not supposing that's going to change. (Though I gave it my best shot as a young man.)

It's easy therefore (for me) to just presume that most people are like me. Evidence demonstrates otherwise, however. Very good data shows that bisexuality is a common human experience, and that attraction by gender is often fluid over time.

It doesn't appear to be consciously fluid, however. Meaning that it doesn't seem like people can just decide what their attractions are.

And speaking of unconscious, I'm sure many men in our culture are consciously quite loathe to think about or acknowledge same-sex attraction even to themselves. Homophobia is so pervasive among men and boys that that such feelings often seem (almost literally) unthinkable.

A psychologist might indeed call that process unconscious suppression. In fact, I've read stories by bisexual men who say they never believed they experienced sexual attraction toward other men until they did – and that after they did they realized they always had. They say they were in such thorough denial that they didn't even know how to think about what they had been feeling.

I don't know. Like I say, I lack personal experience. So I just endorse your suggestion that we believe what people tell us about themselves.