I wouldn't go so far as to say that the labels are invalid, even if they aren't necessarily helpful in complex cases. For sure there are people who are exclusively interested in homosexual or heterosexual relationships.
The discussion is made more complex by the idea that sexual orientation can be considered on factors of actual physical intimacy, or on emotional / physical attraction (which are separate things). There's that whole 'men who have sex with men' label, or other efforts to make distinctions along those lines. And that's not even getting into the genderqueer or transgender side of things.
Personally I understand the convenience of the labels but after a certain point they become distinctly unhelpful. It's especially a shame when people who should be most aware of the power of labels to discriminate and exclude are strong enforces of labelling and use labelling as a reason to exclude others; ie, the discrimination that bisexuals face in the broader LBGT community.
I was not aware of this....I guess I continue to learn something every day here.