Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of person that offered marriage],
I am so flattered and feel blessed that you would want to spend the rest of your life with me. But I am not ready to make that level of commitment, and feel it would be unfair of you to pretend that I am. A decision such as marriage is made too lightly too often in this day and age. Making such a decision lightly leads to such incredible pain and heartache, and I would never want to be the cause of so negative and experience.
I hope that you understand my reticence, and that it has nothing to do with you as a person. I hope that you can bear with me, and be patient.