When Cloud and Aeris are on their way to visit Tifa, they stop and hang out in the little playground there. While sitting on the weird slide (wtf is that thing? plus it looks like a safety hazard anyway) they talk about Aeris and how she used to date someone in SOLDIER. When Cloud asks who it was, she brushes it off.
Can you imagine wtf could have happened if she said Zack? I think at this point Cloud's mind is still pretty damn crippled and he probably would've just up and died. Or maybe he would've been like "oh yeah baby I used to be called Zack but I dyed my hair".
Maybe.