Moriarty loves the feel of being dominated, and he has a fantasy he wants Sherlock to fill for him
Sherlock wants to understand, particularly the criminal mind, and it's not difficult to convince him that Moriarty is willing to help him play and explore the role of a rapist, and understands that it's "just clinical".

I want a fully consensual encounter between the two where they pretend Moriarty is being attacked.