You bring up a good point, even if we leave homophobia aside for a moment their are likely many people who's sexuality is pretty fluid yet they can't identify with the "gay culture" so to speak and therefore reject said label. Someone else who identifies as gay may in fact be Bi but its unpopular to be Bi in the gay community so its just easier to say gay. I'm sure these sorts of oversimplification of ones sexuality take place all the time to save time and energy besides its no ones business besides your SO anyway!