Don't forget too that as much as ZaCloud may or may not have been flirting with Aerith, he can do the same with Tifa. I find it irrelevant to the LTD as a debate, because as has been said, at the end of all the choices, he chooses to be with Tifa.
Another direction to push the thread. I find a lot of Clerii using "Cloud and Aerith's date is the default one" as proof. Leaving the fact that the date is Aerith admitting she doesn't know Cloud and knows something's up, and Cloud not really paying attention, is it really the default date?
When you look at the moments that affect the date mechanism, Aerith starts off with more. However, Tifa can get a lead on Aerith before you even meet her (or tie it up, I don't remember exactly). So, couldn't it be possible that Aerith starts with more stats so that the girls would have an even ground?
I know Aerith asks Cloud for the date, so storyline wise the date makes sense to finally happen. I just don't think it's as default as is claimed.
There's also the ultimania stuff there in Cloud's profile for the date it shows a picture of Cloud and Tifa's date, for example.
EDIT: Looked it up. Before you even meet Aerith (as in, before the church, not before the flower) you can have Tifa's score go beyond hers.