I remember reading that Nomura wanted their to be a strong love triangle in the story when they were first planning verything out but it apparently got tossed into the background, so its possible that Seifer and Rinoa were a bit more serious than is implied. Though the dialogue EyesOnFF8, posted give me the opinion that something did happen but Rinoa probably thought it was a grand romance and Seifer thought it was just a summer fling. I could have sworn that Seifer does mention Rinoa during the Dollet mission or had some dialogue concerning their relationship during the times they fight him either in the Galbadia Garden or in the Lunatic Pandora.I could be wrong, its been a few years since I've played completely through the game myself.