I have a male bisexual friend and a female bisexual friend. The latter often gets asked to make out with girls for the very reason you described, because there's a fascination with bisexual women amongst some straight men.
My male bisexual friend gets so much crap, like being told to pick one or the other, cause some ppl would be fine if he were gay, but both he's just selfish apparently. And the idea of a male bisexual being attracted to men and women just shuts some people down, I've seen it, its just awful.
These are just examples from where I'm from, sadly I'm sure it happens everywhere else too. 
The current paradigm just kind of almost everywhere is you are like an extremely chaotic evil male or kind of a pure female (which can just mean almost any female... although a certain degree of "attitude" will probably break that) of some kind or another.. because patriarchy and all that.
Everyone and everything (and I do mean everyone and everything) that doesn't fit in those, very precise, exact categories is assigned second class status. Including slightly good males and slightly "evil" females, or pure chaotic evil females and pure 'good" sort of males, just everyone that isn't in that exact area.
It ends up working out badly for everyone, including the patriarchy and those females oftentimes.... unfortunately.
Moreover people who don't even agree with it find themselves subconsciously supporting it because they are primed to support that logic.
That said I don't really see that as the pre-eminent, predominant paradigm everywhere in 2016, simply controlling.
Historically it's been because most people/groups never really had the strength to kind of call this chaotic evil person out on the fact that their very narrow, primitive view of the world is really just theres and not at all representative of the world generally.