I would take a deep breath and forgive yourself. Then sit down with her and a have a "grown-up conversation" about what sex is and how curiosity is common etccc, perhaps get a book. Instead of focusing on telling her sex is "wrong" redirect her towards the meaning of the act, who should have sex etc... you don't want to raise a daughter who feels sex is dirty or bad, as she may end up having difficulty with adult relationships later. I would be clear and concise and invite her to ask you any questions and then leave it at that.
