‘Oh What A Night!’ indeed! John and Mary get married, the case is solved, a life is saved, Sherlock makes his last vow and unintentionally deduces Mary’s pregnancy. Why choose the Frankie Valli & the Four Seasons song ‘December ’63’ , of all the most popular wedding songs? There is no such thing as coincidences Mycroft tells Sherlock/the viewer earlier in the episode, so there must be a reason for this song to be played when Sherlock reveals Mary’s pregnancy.
I think it’s pretty clear that this song points to John’s problem with premature ejaculation.