I liked the scene. They're two teenagers stuck in very uncomfortable positions. Tidus, facing the reality that he'd never make it back to his home, and Yuna, facing her death when she summons the Final Aeon. Each is contemplating the end of all they've known, and neither really knows that the other is in the same situation at this point. The scene is uncomfortable and full of nervous energy, and the laugh is their way of releasing it. It sounds forced and unnatural because it was, and it was supposed to be. I thought it was quite well executed and fit perfectly with the characters and the situations they were in. And then we get to everyone else staring at them and calling them nuts, and that fit too. This sort of thing is one of the things I like about X's characters, and, while the scene may be awkward as hell, I wouldn't want it removed.