Daenerys hasn't heard a terrible lot about what it actually feels like; she's overheard bragging and innuendos, but no real descriptions. Maybe there isn't a good enough way to, or maybe it feels different for some people, lots of people don't get to lay with someone they feel this -- this connection to. That must be it.

The absolute instant Jon begins moving, though, Daenerys keens, her hands traveling up and down his back. She bites her lip, getting used to the rhythm of him and the way he fills her.