Sherlock and John have at last realised and revealed to each other that they love each other desperately and have got to the point in their new relationship where they are about to have sex. Sherlock has never done this before and while he trusts John implicitly he is still a bit worried. What if, through his lack of experience, he fails to please John and John is disappointed? So, he asks John to talk him through the experience, so that Sherlock knows what is going on and what he should be doing and what John is going to do to him, so that he can perform adequately.

Sherlock being Sherlock and clueless in this area is requesting and expecting that John will explain clinically in scientific data speak and medical terminology as they go along, and John, knowing Sherlock, knows this is what he’s expecting.

So of course John turns the tables on him and instead guides Sherlock with endearments and compliments and hot, sweet, dirty talk that has Sherlock completely turned on and adorably vulnerable, both emotionally and physically, and leaves him realising that sex is a lot more than inserting tab A into slot B, especially when you are in love. And that John is amazing and perfect, in every possible way, for Sherlock.