As someone who's seriously squicked by somnophilia—unless it's previously discussed and agreed upon, by all parties involved, either explicitly or implicitly—my humble interpretation and opinion is that this behaviour was not, at all, considered outside of the norm for them and what they get up to in the sheets.
What we know: they've been sharing a bed for quite some time now, taking into account both the Before Breaking Up and the After Making Up; they are very open minded and kinky and freaky when it comes to sex, especially with eachother; their compatibility has been shown again and again to be off the charts and they have historically very much accepted and enjoyed any and every sexual act they try together.
Also, in their own words: "Sometimes I heard you moaning while I was half asleep. And it happened more than once." Shin is clearly aware of Jake masturbating in their bed while Shin's half conscious, right next to him. He'd know when Jake is watching him as well as if Jake were to be touching him in some way or another, I would assume. He does not seem, to me, the slightest bit upset or uncomfortable about this—very much the opposite, to be quite honest. He does not tell Jake he shouldn't have done it, nor to not do it again in the future. "So you knew." "How could I not? With a man lying beside you with a bullet hole in his gut... you certainly had no trouble enjoying yourself." Shin says he couldn't not know—most likely he could feel/hear/see Jake. He could have stopped him/told him to stop any of those times. He did not. What he does is needle Jake about having a good time on his own. "Your recovery came first. There was no point pounding your hole if neither of us could enjoy it, right?" These lines, I think, fully speak for themselves. Nothing to add. Jake Gillan, you have never let me down. ♡ "You're insane. What happened to all that talk about there being plenty of other ways, even if we couldn't fuck?" Here I would go so far as to dare to infer they've broached this subject more extensively at least once before, about all said ways they could be intimate without penetration. If not in the past then definitely and without a doubt during their convalescence. Could as well just be my beloved headcanon, however. Regardless, there is no way these horndogs could have been completely celibate, they've been making do however they could—hands, mouths, Shin's armpits, Jake's tits, et cetera. Whatever works for them. Which is a lot. One's body is the other one's playground. "Holding back beside you while you slept so peacefully was actually... quite entertaining. I even rubbed my cock... against your lips. Guess you didn't know that. You took it just fine, even in your sleep." "You're such a pervert. Just like you." Jake's guessing so he's not a hundred percent sure whether Shin knows or not about that one particular detail. For his part, Shin does not confirm nor deny, only expresses that it certainly wasn't a surprise to him or unexpected coming from Jake. Once again, he doesn't come across as anything even close to mad or displeased with that happening, much on the contrary. All the while during this discussion they're bantering (as usual) and teasing one another (as usual) while making out passionately (as usual). "But you didn't hate it." They can, by now, read the other pretty well, inside and outside of the bedroom. Jake's take, from Shin's words and reactions, is that Shin didn't dislike what he did in the least. In fact, it only looked like it confirmed what Jake already believed to be true. In response, Shin rips Jake's shirt open at the chest and climbs him like a tree, proving him exactly right. (So if there was to be a vote pro or against... I'd say we have an enthusiastically positive answer from Shin himself.) Then they proceed to be insanely hot and paw at eachother like hungry maniacs. Some things truly never change.
There are instances when not every single thing is spelled out in a fictional story. When we must read between the lines, rely on context cues, take what prior knowledge we have of the characters and believe them to be true to themselves. So, personally, that's my hopefully realistic reading of the situation.
Cannot stop rereading, they look so fucking excited and gorgeous and happy, what bliss.
Every chapter I think these two cannot possibly get any more perfect and/or freakier... AND YET!!!