Look, you're the one who clicked on this link. I can't be held responsible for what you might read. Back out if you don't want to see all my thoughts about Light Yagami masturbating. (I have a surprising number of thoughts about Light Yagami masturbating, because I think it's a very entertaining topic.)
A while ago, I saw someone in a YouTube comment claiming it’s canonically implied that Light jerked off onto L’s grave, and I am obsessed with this concept. I don’t think it’s true? But maybe it should be?? It's definitely unsettlingly believable.
That said, I’m not sure Light actually knows how to masturbate. I love that, when hidden cameras are installed in his room and Light is trying to seem normal, he apparently thinks, ‘What would a normal teenage boy do? Sit fully clothed and motionless on his bed and very seriously read a soft porn magazine from cover to cover, with an expression that says “hmm, this is some mildly interesting soft porn”.’ Good work, Light; you nailed it. This is definitely what your peers who aren’t murderers are doing.
I do wonder about a scenario where Light bites the bullet and actually goes for it in front of the hidden cameras, though. Light ferociously tries to stay focused on masturbating in a way that doesn’t make him look like a murderer, while also not looking like he knows someone’s watching. Ryuk provides unhelpful commentary. L watches. The way Light masturbates doesn’t look particularly like a killer. But... perhaps it’s too unlike a killer?
I don't think L would get any sexual pleasure out of watching, but I do definitely think that he'd watch the footage in search of clues, and that Soichiro would (understandably) be deeply shocked.
Obviously, if Light actually did have to wank to throw L off his trail, he'd get himself in the mood by thinking about how great he is. Light/himself is the only true romance of Death Note. Nobody else could ever meet his standards.
I'm sorry. These ideas are in my head, and they have to go somewhere.
While we're talking about Light in below-the-belt terms: I'd forgotten Light's brilliant plan of 'I'll give up Death Note ownership and shove another Death Note down my trousers to ensure I don't lose my memories', and I absolutely lost it laughing when it happened on my last Death Note rewatch. Just going about his life! With a supernatural murder notebook down his trousers! For two weeks!
I hope at some point Light was going to the loo, the Death Note fell out of his trousers and he lost his memories. I can picture Light just staring at the notebook, going 'wh... why was there a notebook down my trousers? did someone... put it there? surely I'd have noticed?' until he eventually picked it up, regained his memories and shoved it back down there, furiously embarrassed.