This is pretty much my take, with the caveat that I'd define "flexible" a tad more narrowly: as someone who's sexually-attracted to only one sex, yet can be legit OK about boinkin' the other (i.e., it's inoffensive/agreeable-- the physical sensations are reasonably-pleasant, making another person feel good is genuinely enjoyable, etc.-- despite their maleness/femaleness). They're perfectly comfortable with it, in other words. So, it seems to me, this is meaningfully-distinct from either being bi OR a monosexual who's just experimenting/traumatized/attention-seeking/strictly for-pay: the former are sexually-attracted to both sexes, the latter are-- with respect to the sex they're not oriented towards-- either indifferent or grossed-out, erotically-speaking.

Be that as it may, though, the bi-erasure potential of this concept DOES concern me. Specifically since it might play into the common misperception that "bi = 50/50", rather than attraction to both sexes regardless of degree, so anything else gets you "rounded up" to gay or straight. Will this just exacerbate that tendency? Provide two new options for blotting out the bi-ness of Kinsey 1-2s/4-5s?

I think that, even if "flexibility" is a thing, we've still gotta be careful of this penchant for monosexualizing bi people-- viewing them through that lens; trying to make them fit into that framework. Sure, it's understandable (the majority always assumes that their reality IS reality), but it's also untrue, unfair, and ultimately undoable. So whether it actually exists is one concern; how it's used (as, potentially, the newest tool in the mono-izing toolbox) is another.