Ah, a shipping thread! I haven't seen one of these for a looooong time

Honestly, I always got the impression Aerith was just flirting with Cloud; I mean, they have a few romantic moments in the game, the playground scene (during which the conversation drifted towards Zack) and the Golden Saucer scene (which is optional) being key points in their relationship, but nothing else in the game had them doing anything more extreme than friendly flirting.