Koibito is, in essence, Lover, or girlfriend (or boyfriend for you ladies), with the embedded implication that the relationship is a mutual one.
Even when used without mutuality, such as "Cindy Crawford is my koibito", the word is being used in error, same as if I said "Cindy Crawford is my wife". Both statements are factually untrue, but neither changes the meaning of wife or koibito. It would still be valid to say that Cindy Crawford is a wife, and she is a koibito- to her husband.
But aside from the linguistic koispacebito argument, there's the simultaneous 'and it could refer to anyone' argument, which always told me they were never quite sure about their ground WRT that word, since they're saying 'it has other meanings!' and 'She's someone else's lover!' at the same time. But let's set aside the linguistic side for a moment. Let's focus on the 'It could be anyone!' nonsense. Assuming briefly that such statements (Tifa is someone's lover) is not an utter red herring to AC, her lover would logically be one of the men- more logically one of her friends and allies- seen in the film. Now, among these options, she has expressed interest in only a single man, only one of them has expressed a romantic interest at her, only one of them is described as living with her, belonging with her, only one of them states he plans on starting his new life with her by his side (more specifically, that his new life will succeed where previously he had failed because he had her now, and would have her from that day forward), etc. etc. etc. Just apply occam's razor and cut away the extraneous elements, folks.
But this brings me to another common complaint I have with C/A arguments. Complete and utter lack of parsimony. Parsimony, if you are unfamiliar, is the logical and scientific principle regarding the usage of Occam's razor, which states, in essence, when faced with two or more competing theories which otherwise explain a given set of data, favor the theory with the fewest number of additional assumptions, so that you are left with the simplest adequate explanation.
In practice, this means that the more assumptions, guesses, and ad hocs your explanation has (more accurately, that it relies upon to support itself), the worse off it is.
The C/A is canon argument proceeds with the assumption that Cloud loves Aerith. In brief, conclusion assumption is circular logic and one of the most aparsimonious things you can do.
That, or they point to her death scene (and only her death scene) as the first indica of 'his feelings of love' for her, though I don't believe I've ever gotten a straight answer why his reaction to her death means love, though I do recall a lot of 'Oh, he cried, he said Sephy didn't matter, he didn't attack sephiroth (nor did the rest of the party. Are they also in love with Aerith?)
The other big problem they have is inventing of the ptolemian epicycles in order to avoid C/T evidence. They spend a lot of time spin doctoring C/T aligned quotes, and making up excuses for the lack of C/A aligned ones, and never seem to realize what the situation is telling them.
Then comes their double standards, and I've got a whole different harangue on that one, complete with discussion of various standards of proof and no matter how rigorous your standard, when set even, it keeps coming out more in favor of C/T.
But I've been rambling long enough. Perhaps later.
At least, somewhat that way. I'm just sure I read it somewhere, and it's not a Clerith site either. 
That's true. And you should up and go to sleep already 
I only got barely half of what you're saying. LOL
