Until one day a straight person you know goes off with someone of the same gender and you realise that they were bi.
Well I personally know a couple of people who have done that. I also do not exclude the possibility of it happening to me one day, but we are talking about
propensity(an inclination or natural tendency to behave in a particular way). The fact that there are a few people who have bucked the trend does not mean that the general point is false. Last year Manchester City had a propensity to win their football matches. That does not mean they won every single one.
In case you hadn't noticed, bi people meet a life partner that tends to put them in a box. When that ends, things open up. See Tom Robinsnon for example, He didn't suddenly stop being glad to be gay.
I’ve seen Tom Robinson several times actually. However, his existence does not alter the fact that straight people have a propensity to take partners of the other sex and gay people have a propensity to take partners of the same sex.
As I said before
it shouldn’t matter.
My experience is that most of us have a degree of fluidity.
But most of us have a propensity to lean one way or the other.