Sherlock first sexual experience was a violent gang rape when he was youger. Not only does he still suffer from mental scars, he has physical scarring on his genitals and scar tissue inside his anus making sexual activities sometimes uncomfortable, sometimes painful and rarely enjoyable. On top of that, arousal can be very much a mental issue for him still. That doesn't mean he can't fall in love with John and Mary.

They tried at first to coax Sherlock into deeper intimacy, but they let it go when Sherlock gets too uncomfortable. John and Mary aren't stupid. John has seen some of the scars, although to what extent, he is unsure. Sherlock takes a backseat during sexual encounters but a part of him wants to be able to participate.

Sherlock hardly masturbates and if he gets aroused, he tries to ignore it and suppress it. Slowly, John and Mary get Sherlock to become comfortable with his body again and receive some sort of pleasure despite his memories and pain.