It wasn't about what bedroom they slept in at any given time. Yes, they almost always slept in the same bed, but they also needed their individual spaces. They had never considered the logistics of living together in the tradition sense, because traditional didn't exactly define them. Nothing really defined them, because Jack seemed to have a moral opposition to definitions. It drove Ianto spare sometimes. Jack may have been from the future, where they didn't observe the quaint categories of mere twenty-first century humans, but Ianto was one of those twenty-first century humans, and sometimes he needed the reassurance of a definition. The longer they went without qualify this thing between them was just more time for Jack to pick up and disappear. And maybe this time he wouldn't come back.
But he knew better than to push Jack on something like this. He wasn't going to risk what they did have by pushing for something more. What they had was enough, even if sometimes he really wanted to know what exactly Jack thought they had.
"We do share one," Ianto agreed. "But not in the context Gwen was implying. She thinks we have one room that we both sleep in and keep our belongs in and...cohabitate in. Like a couple." Which he'd always thought they were, but this whole issue was making him wonder if Jack felt the same way.