Aerith dated Zack back when he was alive. When she meets Cloud, he's a big mishmash of false memories and Zack's personality, so he reminds Aerith of Zack. She likes him from the moment they see each other because of that. She had no idea Tifa even existed for a while, and she DOES ask about their status and Tifa flat-out says they are just friends. She also asks Cloud's opinion of her romantically, but that one is up to the player to answer. Still, he's not dating Tifa, and that means he's free to do what he wants, so she decides it's ok to make a move.
Once she dies, she and Zack, her self-proclaimed first love, are reunited. There's really nothing slutty about this... I think it's all rather romantic actually :[