I'm sure someone's already pointed it out, but...

haffieliesel:

Talking about Sherlock’s sexuality in the greenhouse…aka glasshouse in Victorian Sherlock with his ex army “close friend” Dr Watson.

In the British Army when homosexuality was banned, and a criminal offence, those convicted of buggery were normally sent to military prison barracks. They were known as…the “Glasshouse” (originally after the Aldershot one, which had a glass roof).

So discussing sexuality at a time when to be gay was a crime and gay in the army and acting on it doubly so, seems highly appropriate to be taking place in a glasshouse, because that’s where Watson would have ended up if he acted on his desires.