Apparently there's a male lactation joke in the Ferrell/Reilly Holmes and Watson movie. This came out of a conversation with tei on Twitter, and, welllll....

Cis male lactation is a thing that can happen. Nutritional imbalances, hormones, nipple stimulation, all these things can cause it under the right circumstances.

Imagine if Sherlock, overrun with guilt for his role in inadvertently, indirectly, causing Mary's death (and is not rejected as harshly by John, so doesn't resume hard drug use) feels very guilty for having played a role in robbing Rosie of a natural healthy food source.

Imagine what he might do to try to rectify that (without asking John's input first, of course).