John is straight, he loves Sherlock, but is completely uninterested in him sexually and prefers to sleep with women. Sherlock on the other hand is extremely interested in John sexually and believes if John would just try it, he'd like sex with Sherlock.
Someway somehow, John and Sherlock end up in bed together and Sherlock fucks John into the mattress. Sherlock is very skilled and manages to make it physically pleasurable for John, but John isn't entirely comfortable with it mentally. Which results in a personal crisis for John.