Wasn't Tifa added in as an after thought? when they decided to kill Aeris? that might be speculative or flat out wrong but I always saw Tifa as the secondary love interest, but who steps up to main love interest when Aeris dies.
Plus Cait Sith said Cloud and Aeris were more compatable or something, why bother putting that in if Aeris wasn't the main love interest?

I dunno, this is a question that can't be answered. I'm resigned to thinking its just "complicated" like love in genereal