there are lots of headcanons about sherlock speaking french during sex and john dying because it’s so hot, but listen. what if sherlock speaking french sounds like martin crieff speaking french. what if it’s not sexy at all, it’s hilarious and john loses it and laughs hysterically and sherlock scowls and sulks and determinedly keeps his mouth shut during sex because he doesn’t want to embarrass himself but he just can’t help it and it happens again and again
This is absolutely how it would go, which is why I’ve never once done a Sherlock speaking French head canon.
Also John would bring it up at other random times just to mess with him, like in the middle of the dairy aisle at Sainsbury’s, suddenly John would turn to Sherlock and be like “allons -nous obtenir un peu de fromage, mon amour?” In like PERFECT French, and Sherlock would do the chin wobble and then scowl and stomp away and John would double over laughing, trying to call out “Sorry, I’m sorry, I just can’t help it. I love you, I love you, come back, Sherlock,” but still like crying laughing, while Sherlock stands outside and sulks.