Never made sense, just like most FF romances. She never seemed to be too close to Wakka in the first game. She was plainly pretty cold to him, even if they were still friends (sort of). They just went from friends to parents with no real build up. It seemed like Square figured every one had to have some sort of changes in their lives so the only two characters in Besaid might as well get it on because we can't think of anything else they could do.