One of my pet peeves is when people mix up the following things:
1. Being between orientations
2. Fluidity in orientation
Because I am gray-A, I am between orientations. That does not necessarily mean that I am fluidly switching back and forth between allosexual and asexual. It's not like I'm asexual for most of the year, but the local ace meetup group has to avoid scheduling on full moons. No, I feel sort-of-not-really-attracted to people on a daily basis.
Of course, some people might experience fluid orientation. If someone's level of attraction varies from month to month, that's a pretty decent reason to identify as gray-A. If someone's level attraction slowly varies over their lifetime, I'd say it's up to them to choose which labels are most convenient at which times.
Wouldn't you know it, this is a confusion with bisexuality as well. I think the Bisexual Index put it best:
Often you'll hear long winded definitions of bisexuality include the word 'fluid', or 'changeable'. Some bisexuals like the word, because it feels to them like their sexuality does change over time. One day you might be only fancying long haired people, the next week all your fantasies might be about office workers, or pizza-delivery-people. Or you might not - some people have a type and stick to it. That's fine with us. But why do people who aren't bisexual like the word?Likewise, when people assume that gray-A is necessarily about being attracted to people very infrequently, I think it's because they just can't grok the idea that anyone can be inbetween at every instant in time.
Because it explains away the gender attraction - they can't get their head around people liking more than one gender, so they couch it in terms of the attraction changing, flowing, from same-sex to opposite sex and back again. When non-bisexuals define bisexuality as "fluid" what they usually mean is "no-one can be genuinely attracted to more than one gender at the same time, so it must be about being gay some days and straight others".
Next on my pet peeves list: when people confuse fluidity and choice.