"Give them time," she assures him, smiling. "If you keep coming back to me, they will come to welcome you almost as eagerly as I do."

Technically she can't stop him from bedding anyone else, and she wouldn't really want to. That's not what they are to each other. It would still be a bit strange to deal with the knowledge that he might come visit to warm her bed and then leave it to go warm another in Edoras in the same visit.