Girl Friends With Benefits?

So I have a crush on this girl and I thought it would be a good idea to tell her. This is my first girl crush and I love her... she's actually been one of my closest friends over the past few years and when I told her she said she hoped it was her. She and I talked it over and she explained she thinks she only has a physical attraction to me and we are planning to have sex in a couple weeks. Am I making a mistake? I want her love and I want to be happy too. Advice?