Honesty. Explain to her how much your faith means to you. Explain to her that you feel that the bond that you two have is too important, too beautiful a thing to cheapen it by reducing each other to merely sexual objects. That you think sex between two people is such an intimate bond that to do so outside of marriage is to offend the natural dignity that she deserves as a being created in the image of God.
Make sure to convey it in a way that reminds her that she is more important to you than just sexual intercourse, that she is the object of your love, your respect, and your admiration. If she loves you, she’ll be more than happy to accept that you want to cherish her not just as a physical object, but as a part of your own being. That when you two are married, you will no longer be separate people, but joined in a union of immense beauty and love. One that can only be experienced in the Spirit of Truth.