I have a much harder time accepting Wakka and Lulu, who while they were civil to one another during the game, weren't exactly enamoured either.

It's not like Tidus sticks his tongue down Yuna's throat. I don't think he'd do anything but give a big hug to his friend who he hasn't seen for years while he was in the afterlife somewhere.

Also, I could see him having a relationship with her even if he was disconcerted by the changes in her even out of a sense of feeling beholden to her - I mean, potentially the fayth gave Tidus the heads up on Yuna being the one who wanted him back. I would definitely feel like I owed her one.