Sherlock was never really one for public displays of affection. It wasn't that he was scared of retribultion, or gave a shit about what others thought. He wasn't afraid, and he didn't care. It was just not in his nature to be demonstrative in public, unless that public was in an BDSM club. There he didn't mind showing off his prowess. He could be quite a nice both voyeur and a exhibitionist, whichever the scene called for.

He tensed slightly at the first touch, then relaxed into it, slipping his fingers between John's and giving him a little squeeze. It wasn't much, but it was enough contact to show their togetherness.