You. Me. Handcuffs.
Handcuff (n.): A pair of lockable linked metal rings for securing a prisoner’s wrists.
For his first encounter with River Song, the Doctor is claimed as hers, with a ring unbreakable; her handcuffs. As their encounters unfold, the bind softens: steel, leather, silk, bio energy, at last flesh, when he stops River’s hand from slapping him. The Doctor is, this time, the one to ‘handcuff’ her, one hand, as she did long ago in the Library. The first handcuffing had seen River disappear just after being introduced. The second would give her reality for a second, for a kiss, before vanishing again. River had punched him then, depriving him shortly of the world and her presence, buying time. He had blacked-out. The stopped slap in the tomb was the Doctor refusing another black-out, this time voluntary and directed at her, as he had ignored her phantom.
If they share a leather wrist band, it is only because they are of the same fabric; time. And time cannot be unwoven easily. It is another exchange of vows, taking place outside time: the Doctor slipped on River’s Vortex Manipulator as the Universe was dying and River did as the story was ended, after the happily ever after. In-between and after, every clutch, steel or stone, would eventually be replaced by a flesh or silk caress. This is a pact, for both of them, to protect another from physical or mental harm. Beside the imprisonment symbol, handcuffs are a link that cannot be broken. And in here the strongest are not made of steel.
The steel wedding anniversary celebrates the eleventh. Bowties may be cool, but the wedding rings were handcuffs in this story.