fantasy rape is like submission/domination -- aka ROLE PLAYING. My husband and I enjoy taking turns on each other. If you are both into it can be a fun way of exploring each other deeper (no pun intended) and can raise your level of trust with your partner. Of course, the trust should be there before you start this, as well.
For me, I think that I tend to enjoy that type of roleplay occasionally with my husband because I am a very controlling person, and for once, things are out of my control. But I've never had any fantasies of someone else doing it to me, only my husband, and I wouldn't go as far as to call it a "rape fantasy" either, more like domination.