Okay so when Mary says this,
it always really irks me.
Obviously at any wedding it is a powerful moment when the couple refers to each other using “husband” or “wife” for the first time. However the power shown here is not one of love, but rather dominance. We already know that the relationship of John and Mary is an echo of a romance, a shadow that never has and never will be the actual object from which it is casted from.
So why did Mary refer to John as her “husband” if not for the romantic effect? Well that’s because Sherlock was there.
Earlier that day when Sholto arrived, Sherlock had a small conversation with Mary. Sherlock unintentionally made it pretty obvious that he was feeling jealous about the fact that John appeared to have a close relationship with another man, especially one that Sherlock didn’t know. Mary quickly picked up on this.
Despite the fact that their relationship is built on lies, Mary and John did just get married, so there still is a romantic relationship there. Whether or not it is a good one doesn’t stop it from existing, so Mary is still considered John’s romantic lover. In this gif Mary said that Sholto came first out of a group consisted of herself, Sherlock, and Sholto. Well if Mary is defined under the category of romantic lover, then by saying this that means she is identifying all three of them as being in the group of John’s romantic lovers. So Mary knows.
This brings us back to the first gif when Mary called John “husband” whilst Sherlock was standing right there with them. Mary is well aware that not only did Sherlock have feelings for John in the past, but that he still does to that very day because just earlier he was getting very jealous over John simply talking to Sholto.
But John belongs to Mary now, and Mary has every intention of making that painfully clear to Sherlock. Of killing any leftover spark left behind from the unspoken romance of John and Sherlock. Besides, Mary is an assassin after all. She kills people for a living.
But what is the best way to kill John and Sherlock’s relationship without physically killing one of them
at least in that moment.? By declaring that John is officially hers and that there’s nothing Sherlock can do about it. So Mary tells Husband that they should go and dance now. Which of course would leave Sherlock standing there alone, this time with much more than just jealously taking over him. This time Sherlock is left dead inside.