Sherlock's in love with John and hopes John loves (or at least lusts for) him as well. Sherlock's planning to seduce him. In order to research what John likes he checks the porn on John's laptop. He finds a vast variety of very different porn files from vanilla stuff to hardcore porn, f/f, m/m, m/m/f, multi etc. not noticing at first that the only thing they all have in common is that one of the involved parties (even in the lesbian porn!) looks like Sherlock.

Bonus points for the semblance not to be obvious. John needs to point it out to him. Like maybe one guy doesn't look anything like Sherlock - only his eyes remind John of Sherlock's.

RTYI welcome!