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.
If we're going to go down that route then let me tell you that originally Cloud and Tifa were going to have sex under the highwind, with no mistake for "Maybe they just talked." or anything. Originally, they were going to go into a Chocobo stable, and come out the next morning and Cloud was going to look around nervously or something to that effect.