Bisexual erasure

No, bisexual people aren't "just going through a phase."
Perhaps the reason bi celebrities may be reluctant to come out is that they fear it would hurt their careers in a way that being gay no longer does. Perhaps they fear their fans will wonder whether they are confused, promiscuous, or indecisive. That's what people wonder about non-celebrities, after all.
It's not the label people hate you for. So what is the problem, if it's not the label? It's not fitting into boxes. Not fulfilling your assigned role. Being too queer. Refusing to toe the proverbial line. That's what they hate.
When I came out as bisexual, I heard responses like, "It's just a stepping stone between identifying as straight and identifying as gay," and, "See how you identify in five years." I learned that if a person does not fit in the straight-gay binary, some people won't believe them.
I tell you this: Every time I read an article about bisexuality and the Kinsey scale comes into it, I see red, because I know that I'm about to be misrepresented as a divided person rather than an integrated one.