I can understand how monogamy and bisexuality can be seen as being at odds: if you have to choose one person to be with, then there is a choice being made between the sexes ultimately that you have to make.
Unless you happen to land someone who is medically designated intersex, or in that ballpark. At least until someone in the medical industry finally caves and admits that so very many
birth certificates have long been based on wildly inconsistent penis standards. Since most people are thinking about average ranges of body shapes or certain stereotypical bio shapes of various features: Anyone could
land someone who they happen to interpret as something besides exclusively male or female. All it would take is applying that same sort of fluid principle the doctors have been applying to thousands of penises (edit: and not a few "unusual" vaginas), to other parts of the body -- parts that are more visible in daily life by far -- and noting a few that make one think twice. Since experientially, orientation
as such is not only about what's socially available. It's also about what one is subjectively looking for.
There is the gay gene sort of argument that a certain percentage of object orientation is genetically driven, but that doesn't fully account for the vast range of very specific body types people can and do choose to pick over in hunting for partners. I think that would also logically open up the possibility that any "natural" ability to identify a biological sex might also be confused through others' variations. Plenty of individuals just are not shaped to cue the usual bio-sex readings... Does one "really" like this woman because she's seen as masculine, or was one actually
searching for more signs of a male body per se? I'm doubtful that we have the means to evaluate where pursuit of gender ends and where pursuit of bio-sex begins, consistently across the population. Plus, that is assuming that people actually operate in the way that orientation suggests to begin with: Only making choices based on perceived bio-sex and not based on gender.
Generally speaking: If we had to look for some externally measurable, "legitimate" orientation apart from claims of the person in question, could we tell? Sure, we can strap people down to chairs and measure their responses to images -- Or is that more their response to being strapped down and fed any image whatsoever?
Heh. And then the ethical dilemma! This goes off in various messy directions if you think it through, both scientifically and politically.
Of course what's totally pure and honest reporting for one is not approved as socially "legitimate" according to so many others. Noisy moralizing minorities and oversimplifying, complacent majorities may reject whatever individuals say about their orientation. Particularly when the individuals in question take the more politically threatening choices, such as bi.