Things happen the other way around: during her and John's wedding Mary realises that Sherlock is pregnant. She also realises that the baby is probably John's (which it is, indeed). Maybe it was drunk sex that got Sherlock pregnant. But neither John, nor Sherlock himself knows about the pregnancy yet.

A/B/O with Omega!Sherlock and Alpha!John or just MPreg AU where some men can get pregnant, I don't care.