The only sex Sherlock has ever had was with Janine's big penis up his arse, usually with her full breasts pressed against his back. She doesn't have sex any other way other than topping. She's also quite dominant and sometimes a little persistent when she wants sex.

Maybe she's not a really dark character, but she likes to take what she wants.

Unrequited John/Sherlock (unrequited on John's part) is very welcome.