when they start having sex it’s a bit overwhelming for sherlock. this isn’t just sex, this isn’t just sex with two people. this is sex with mary and john, who love him and who chose him.
they’re constantly encouraging him to keep talking and say what he likes, what he dislikes. they explain every new thing they’re going to try, make sure he’s comfortable. john in particular wants to check that it’s okay constantly - mary knows he’s a big boy, but john can’t forget the times he’s seen sherlock at his most alien. where does he want to be touched, what does he want to do, is it okay if john just watches him and mary this time.
and though this is a bit of a new thing for him - he’s had relationships, thank you mycroft, but they were mostly for data collection to see what all the fuss was about - it doesn’t stop being him sherlock. when john says something overly coddling or incredibly obtuse, sometimes he can’t hold the sarcastic remark in and it just comes out sounding, well, sounding like sherlock, and they banter and niggle, and sherlock will tease john and bring him relentlessly to the point of orgasm. he’s a quick learner and never going to let john forget it. john can take him down a peg but sherlock gives as good as he gets. that’s how they work so well, that’s always been them.
but with mary? sherlock is the bottomiest bottom to ever bottom.
maybe it stems from her being so much like him, but better, so much more human - so much more everything that he’s wanted to be all his life and was always told he wasn’t, that he gave up trying to be, what they bring out in him - that he has such respect. maybe it’s because she is truly astonishingly wonderful, and she lets him share john. or rather it’s because she’s john’s, and has been a window into their bed and their home and their heart. but she’s all of these things, she is a treasure and when he looks up at her riding him he can see starlight bleeding around her edges because she is an absolute marvel. he is in awe. he is her’s, utterly and completely and unquestioningly in this moment.
mary gives him instruction in a gentle yet uncompromising tone, smile so sweet butter wouldn’t melt in her mouth. sherlock gulps and nods and looks at her with pupils blown wide in arousal, and struggles out a broken, “yes ma'am”, from a suddenly dry throat.
