A thought can’t hurt anybody either. That’s the whole point. People may think of killing somebody…but if they don’t do it. there is no victim.
Where did you get the notion that for something to be wrong it has to ‘hurt’ someone. Then there is the case of physical hurt, mental hurt, verbal abuse, and then within that the degrees of hurt.
I specifically brought this out, because the OP mentioned “sin”.
Leaving aside the religious connotations, the secular society generally sees “sins” in public, even though they aren’t of a religious nature. Admitting to paedophilic thoughts is one of them, that will immediately send warning signals throughout society.
No one can be prosecuted for it, as it hasn’t gone into action, however the concept of sin still exists, which is deemed wrong.
For something to be wrong, it doesn’t have to be killing.
There is also the fact that excessively thinking of certain things and scenarios
may eventually lead you to end up doing it, this has some basis to it. Such as thinking of depressing thoughts all the time will no doubt lead you to live a depressed life, having murderous thoughts may lead you to end up losing control at some point whilst in an angry mood. Though whilst in thoughts you aren’t inflicting damage, you are setting up a pretty good pre-requisite for trouble should a scenario arrive.
Whilst in the masturbation/thoughts scenario, this may not apply, your reasoning is faulty that if it doesn’t affect anyone else then it isn’t wrong.
What if it affects the individual doing it, in a mental sense, and he doesn’t realize it, and suffers over time, is it still wrong then? Or does it depend on realization? Perhaps it’s subjective? Then maybe actions aren’t wrong either then, infact who is to say I’m wrong to say…kill someone with no reason at all? Would it be society? and where did they get this idea of “wrong”, the law? Where did the law get it from? I could be here all day, but you see the point.
What is wrong with masturbation is, not only can it become an addiction, it reduces another human into an object solely of pleasure, you may not be physically hurting them, but what we can be sure of is it ultimately reduces the individual’s thoughts to look for people who can generate the most amount of lust in them which can be a failing point for marriage later on as one’s standards for a woman has been distorted into a purely sexual sense.