The top comment is almost spot-on. (I say almost because... well, I don't agree with the latter half.) "Sexuality can be fluid, but it doesn't mean that it (always) is."

I personally do believe sexuality is fluid & that technically every human is capable of that, as every human is technically of spontaneously developing a tumor. It's unlikely to happen in many cases, and I'm not saying that it's likely that Mr. Gay Is My Identity 100% Penis Only Supergay is going to wake up one day and realize he's now straight.

However, I never think this should be used as an argument to convince people to become bi/pansexual willingly or that everyone is inherently bi/pansexual and anything else is bigoted. Despite sexuality being fluid, I don't think it's a choice nor easily influenced. I believe it's environmental, and using myself as an example, I fully believe I technically have the capacity to be attracted to women, however due to environmental and possibly neurological causes when I was young I believe I turned out homosexual, and that's something that's proven very hard to undo.

I dislike the Kinsey scale, though. I think it's useless and doesn't account for people who's preferences do switch (something I hear isn't uncommon in bisexuals) or people who don't follow a linear scale entirely. Sexuality is a very complicated matter and attempting to "diagnose" it with such a simplified scale is pretty much useless. So I can't help but be wary of people who claim they are a kinsey something because it kind of gives an air of overconfidence yet little understanding to how sexuality works.