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Sherlock orgasms when John refers to him as 'husband'. Established relationship, romantic, filthy-explicit married sex.
If, a year ago, someone told John that he would be on his back getting fucked enthusiastically up the arse by Sherlock Holmes, he would have decked them with no hesitation. If this hypothetical person had added that he would also be begging for it like a wanton slut, he would have given the first punch an encore. But that person would have been correct.