You can't see them. Sherlock can't either but in a way he does.
He imagines Mycroft's to be leather, kind of like a leash, strong enough to withstand the occasional pull.
His mother's were cold, hard metal, biting into his skin when she yanked at them.
Mrs Hudson's are like lace, softly woven, meant to help. They tear when he doesn't react in the right way and then he has to deal with the disappointment on her face.
And then there are John's. He sees them as velvet with a core of steel, guiding him, stopping him when necessary, but never hurting. Sometimes they take the form of words, but most of the time it's a glance, a cough or – his favourite – a soft touch to his arm or the small of his back.
I am talking about restraints. And I don't mean the obvious ones. They do those too. Took Sherlock a while to get used to giving up control but in the end he discovered that he really liked it.
No, I'm talking about the restraints people put on Sherlock. His morals. Mouth. Brains. Behaviour.
Everybody does, even if they love him like Mrs Hudson, Mycroft and John do.
The point is: be it metaphorically, sexually or romantically, when it comes to John Sherlock does not mind being bound.