Curvature.

There's just something about the curvature of Rory's spine that always sends the Doctor over the edge. He doesn't exactly know what it is; whether it's the sheer feel of it underneath his hands, the soft skin stretched over each individual vertebrate or the way his hands fit perfectly, conforming to it while they caress and scratch and bruise. He doesn't know if it's the way it arches in such a position that he can apply math equations to it or if it's the way it ripples when the Doctor does a certain thing, moving in sync with Rory's moans or if it's the way that it fits perfectly against the Doctor's chest afterwards, one part of a the puzzle that is their bodies.

To be honest, the Doctor doesn't care that he doesn't know which of these reasons is the right answer. All he knows is that the most beautiful thing in the world is the curvature of Rory William's spine, whether it be curving away from him or towards him or whether it's below him or above him or beside him or whatever. It's just perfect and for the first time in his nine centuries of existence, the Doctor is content to just leave it at that.