My opinion.

Start slow, do not expect to jump right into this. Have your fiancee act out the scene, and expect to be frightened. Tell him to stop at this point. I know this will spoil the scene right then, but it is better than being tramautized.

Then do it again another time. Eventually you will be able to act out the whole scene. Then you can get lost in the fantasy. If you ecpect this to work the first time, you will be disappointed. It rarely does.