If you omitte evidence from FFX-2? Then, it was put there as a fanservice, so that people could write fiction or not be forced to feel that Yuna no longer has Tidus as an option. At the time of writing the script of FFX, there was no intention of creating a sequel.
Actually i do believe that it was said that during the time they were still making ffx, there were intentions (discussions) of making a sequel. Correct me if i am wrong though.