Mounted by a Monster: Doppelganger

Mary's billionaire boyfriend is perfect in almost every way, except for their utterly boring sex life. She has finally convinced Brandon to try a little fantasy role-playing. What Mary doesn't realize is that a doppelganger has taken the role of Brandon in tonight's game, and it's no longer make-believe. To Mary's delight, she discovers that the doppelganger doesn't play anything like Brandon. This paranormal erotic short story is about 4, 100 words.