“Oh, have you had sex?”
So some people are reading this as Eurus deducing from Sherlock’s playing (of Irene Adler’s theme) that’s he’s had sex. Which is a very plausible reading.
But I think it can also be interpreted in the opposite way.
We (the viewers) have heard a lot about Sherlock “the virgin” who “wouldn’t know” about sex, so we think it’s self-evident that he’s virginal, and it would be surprising if he weren’t.
But Eurus hasn’t been around for all that. To her, he’s just a 40 year old man who has *not* been raised in an institution. It would be natural to assume he has had sex at some point. So quite possibly her deduction based on his playing is that he *hasn’t* had sex, and that’s why she asks the question.