The Sign of Three visually tells us that John swings both ways.

So I was going through all the screen caps of a Sign of Three to find the scene where John is visualised by Sherlock inside his mind palace because my first reaction when I saw him there wasn’t ‘Oh hello John’, it was ’Well hello John that’s a rather bright cardigan for you, since when did you wear cerise...’

And then last night I realised if anyone was going to tell us the truth about John’s sexuality, it’s going to be Sherlock.

Well maybe.. but in the course of looking for this screen cap I found these ones instead and took a moment to stare in awe at the last club scene one of the stag night as I realised in fact it’s with John himself we see into his own mind regarding his feelings for Sherlock.

  And now let me give you this.

                             This is the bisexual pride flag. And John in the last scene in the club is literally standing in front of this flag. Right you’re going to argue that the lighting is the wrong way round, but this scene is being rather subversive and also playing a very clever trick in how it becomes lit, in regards to John’s thought processes…

Because each of those colours represents what bisexuality means.

The deep pink or rose stripe at the top of the flag represents the possibility of same gender attraction; the royal blue stripe at the bottom of the flag represents the possibility of different gender attraction and the stripes overlap in the central fifth of the flag to form a deep shade of lavender or purple, which represents the possibility of attraction anywhere along the entire gender spectrum.   source

So once again let me break it down…

Prominent blue. Streak of purple and a hint of pink.

This is John at the moment he decides to knock that sneaky one down. This is John Three Continents Watson the ladies man…but the pink is creeping in here, the same sex attraction…

And now that pink lighting has become more prominent and John is sneaking that extra shot into Sherlock’s beaker WHILE looking at Sherlock. Pink equals same sex attraction remember.

But look how the lighting is hitting Sherlock…

He is now shaded in the purple and pink light and the blue is stronger on the other side of the screen. We’re looking at Sherlock through John’s eyes here, remember and John does not consider himself to be gay.

But evidently John is attracted to Sherlock and he is also attracted to Mary. But that is why there’s the purple stripe in the flag. It means this person is neither gay or straight because their sexuality is between those two states. And that’s how John Watson swings both ways.