Sex

Sexual intercourse (or dancing according to Moffat) is an activity completely shrouded in mystery to /who/ due to unfamiliarity. The mechanics of the process are not wholly clear. They seem to involve appendages and holes, while participants seem to holler in pain together until they cease due to exhaustion.

The idea of the Doctor interacting with the idea of this custom enrages /who/ because it distances them further from the character, prevents them from projecting their loneliness onto him. Luckily, the current Doctor is thankfully an autistic sexless creep and is consequently the best Doctor ever because she's the most relatable one for us.

Not always voluntary, as evidenced by the current state of Doctor Who.