John once went down on Sherlock and sucked him off. But he said "this is mine" or something similar and Sherlock took it as John saying Sherlock is not allowed to orgasm unless John is there to make it happen.

Then John leaves for a few days - a week for some conference or whatever reason John and Sherlock can't be together.

And Sherlock doesn't masturbate at all even if he wants to release tension. It all builds up and his testicles swell. Sherlock is horny and its started to ache!!! When John comes home, Sherlock is all tense and tender and John is slightly confused but amused. Of course, he brings Sherlock pleasure and forces multiple orgasms out of him until Sherlock's almost positive he can't possibly release any more for the next month. Then of course, John fucks Sherlock into having more dry orgasms.