Feeling lazy, Sherlock asks John to give him his violin. John is exasperated with him, but, as usual, does what Sherlock asks him to. Carefully, he takes the instrument in his hands... and, as Sherlock sees it, he suddenly gets a mind-blowing erection. At first he thinks it's some sort of an accident, but when it happens again, his body reacts the same way (he even comes in his trousers and becomes very vocal, which gives him away to the very stunned John). It just works this way, John touches the violin, gently runs his hand over it and such, and Sherlock gets aroused or even comes. Sherlock is otherwise mostly asexual.

It's great if John and Sherlock are in an asexual relationship (but they can have sex anyway sometimes, because Sherlock doesn't mind and John is sexual), but now that they know about that violin thing they can experiment. They can also experiment without having a relationship before (in that case I prefer virgin!Sherlock), it's up to you.