It's only when Jack asks him to stay that Ianto realizes this... thing they have might be more than just a convenient arrangement. Despite the subtle hints they've both been dropping for the past few weeks, it's that definitive action that finally convinces him that he's more than just a way for Jack to get off easily.
The action itself does come decidedly out of left field. It's late at night (or early in the morning, depending on your perspective) and he's basically lying on top of Jack, their skin sticking together from the sweat that has yet to evaporate. Ianto is still waiting for his breath to come back, gasping into the nook between Jack's shoulder and neck, his eyes closed. For once, Jack is keeping his snappy remarks in his mouth; instead, he's panting into Ianto's ear, his hands splayed out over his bare back.
Ianto can't help but feel a little proud that he actually made Jack speechless.
Despite this sense of pride, as soon as he's able to properly breathe and think about something that isn't Jack (or the way Jack had moaned when he'd cursed in Welsh or how amazing those hands of his felt pressing into his hips or...), he rolls off of the small bed and starts gathering the various components of his suit together. He rounds up most of the pieces fairly easily, with the exception of one sock (he'll just steal another one of Jack's) and his trousers, which he eventually finds dangling down the trap hole to Jack's bunker. He starts to ponder just how they got there but before he can come up with a solution, Jack's fingers are around his wrist, making Ianto freeze on the spot.
"Stay." It's one word, just one word but for a few seconds, Ianto thinks that he might have misheard Jack. After running various, similar sounding possibilities through his head and realizing that none of them makes sense, Ianto swallows hard and looks at Jack's face, looking for ulterior motives of some sort.
He isn't sure (he never is) but Jack genuinely looks like he wants him there. His throat feels too dry to speak so he simply nods, dropping his clothes to the floor in a heap before sliding back into the bed. The damn thing is really too small for two people so he's forced to lie with his stomach pressed against Jack's broad back. Considering the various positions they were in only moments before, it shouldn't feel nearly as strange as it actually does and Ianto realizes that he doesn't actually know what to do. When it comes to the sex, he doesn't have to worry about where his arms go or anything like that. But this... well, he supposes it's cudding; the cuddling is all new for him.
"You're over-thinking it." Jack sounds like he's already half asleep and when he raises his arm to flick off the light switch above the bed, his movements are distinctly less graceful than Ianto is used to. Once the light is off, he reaches behind him and grabs Ianto's hand, bringing it up to his mouth and laying a kiss on each of his knuckles.
Ianto had thought he was too old to get butterflies until that moment.
"Just do what feels right," he murmurs, releasing Ianto's hand. Following his advice, Ianto lets his hand go lower until it's resting on Jack's stomach, his fingers grazing over his hip bone. Jack gasps once, the noise almost swallowed up by his pillow and he rests his arm on top of Ianto's, his fingers entangling with his own.
"Goodnight Ianto," Jack whispers in this sleepy way Ianto has never heard before. He decides on the spot that he likes it and that he'll do anything in his power to hear it again.
"Goodnight Jack," he replies, pressing one kiss into the spot below Jack's ear before shutting his eyes. Now that he's used to it, he doesn't know why he was ever afraid of this; why he was ever wary of spending the night with Jack. His mind is still racing with things he feels like he should do before going to sleep; he knows that he should fold up his suit before it gets wrinkled beyond recognition and he knows that he should probably close the trap door, just in case any of the others show up early to work.
He knows he should do these things but, if he's being honest with himself, he doesn't want to. The truth is that there's nothing else he wants to do but fall asleep beside Jack and appreciate the fact that they're both alive.
So he does just that.