How should I talk to my daughter?

Needing some parenting advice. My 17 year old came to me and said her and her boyfriend are wanting to be sexually active. I do not want her to be, but I feel like it was a big step that she came to me and I want to make sure I respond in the correct way. We have had many talks, I just don’t know what to say to her. I know teens are going to do what they wanna do, but how do I talk to her about this? They have been together for 4 years and we love them together but i still wish they would wait.