just-sort-of-happened:

One of the ways that Sherlock deduces that David still has romantic feelings for Mary is that in all his Facebook pictures of John and Mary, John is, ‘always partly or entirely excluded’.  This shows that he wants John, ‘out of the picture’, so to speak.

Then we see director Colm McCarthy’s approach to framing his shots during the best man speech.  During Sherlock’s speech, Mary is, ‘always partly or entirely excluded’, in any shot that also includes Sherlock.  Janine, a character we’ve only just met, and of much less importance to the proceedings, seems to have plenty of room to fit in shots that exclude Mary.

If excluding half of a couple is a sign that someone wants them to not be a couple, then, here, the show is explicitly telling us that John and Mary are not the right couple.  The compositions escalate from merely cutting Mary out to having Sherlock actually physically block her from the audience’s view.  We are meant to not see John in relation to Mary but Sherlock.  It’s always Sherlock.

(Thank you to obliquely-related for their comment that reminded me to write about this.)