Let’s say there is someone who is attracted to children. They do not have to be exclusively attracted to children; it is possible they could be attracted to adults, as well as children. Maybe they just have the capacity to admire beauty wherever they find it, regardless of such mere technicalities as age. Let’s say this individual masturbates, in the privacy of their bathroom or bedroom, to images of children (not child porn or anything; just normal, fully-clothed children in mundane everyday situations). Meanwhile, this person is entirely aware and cognizant that children cannot consent to sexual activity, and, therefore, does not even entertain the thought of ever touching a child. Masturbating is as far as it will go. This person will never, ever touch or harm a child as long as they live. While we can (probably/hopefully) all agree that someone actually touching, molesting, or engaging in sexual activity with children who cannot consent is morally wrong, what grounds do we have for asserting that the person masturbating, as described above, is doing anything morally wrong? In my opinion, none. If someone is a utilitarian when it comes to morality (as I am), then, only actions that cause harm to others (or perhaps to oneself, as well) can be considered morally wrong. And it is clear that someone masturbating to a child in the privacy of their own bedroom or bathroom does not harm anyone. Neither the child nor its parents are harmed physically, and, as long as they don’t find out, they cannot be harmed mentally, either. Meanwhile, the masturbator gains pleasure. Thus, overall, the action has the result of increasing the net pleasure in the world, whilst having no impact on the net suffering in the world. Therefore, from a utilitarian perspective, there is nothing morally wrong with an adult who is attracted to children masturbating to pictures of clothed children engaged in everyday activities in the privacy of their own bedroom or bathroom.
Welcome, OceanWaves. This is a puzzle of a question.
You make quite clear that no pornography is involved and also no contemplated or attempted sexual activity with children. The question is, though, or so it seems to me, why images of even fully clothed children should evoke a sexual response. Because that, of course, is precisely what masturbation is : 'deliberate self-stimulation which effects sexual arousal' (Kinsey, who is acceptable on this point at least).
Purely aesthetic appreciation of children would not normally lead to masturbation any more than viewing a landscape or listening to a symphony does. So there is an extra element, something more than and different from the purely aesthetic, in a masturbatory response to children. That element is specifically sexual.
I don't think we need worry about masturbation as our ancestors did, or some of them, as a threat to the survival of humankind or as leading to insanity, neurosis, neurasthenia. What concerns me about your 'solitary activist' is that I don't think he is realistically likely to have solely the state of mind you describe. If he is sexually aroused by the image of a fully clothed child, isn't he likely to be sexually aroused by one of a naked child or of a child in a sexual position ? He may be fixated purely on images of fully clothed children but you do not specify this. Having people around who are sexually aroused by images of children creates the probability that some such people will not remain satisfied with solitary activity (abeunt studia in mores) : and that does pose a moral danger.
I know I am moving away from the exact situation you described but I don't think that situation is stable; it risks tipping over into the consequences I've unfolded. But if it is stable for your individual, who only ever sees images of fully-clothed children and never imagines or is exposed to child pornography, and is never inclined to sexual interference with children, then I suppose that individual is harmless and relishes the pleasures that (genuinely) come his way.
There is, perhaps, another angle, though. You don't say whether your sexual practitioner confines himself totally to this activity or whether he also engages in sexual behaviour with consenting adults. A sexual life wholly sealed off in masturbatory activity seems likely to be foreclosed to the pleasures and fulfilment that (I venture the view) can only come or is far more likely to come from consensual inter-personal sex. In the calculus of pleasures, my guess would be that a sexual live devoted solely to masturbation is lighter in the scales than one shared with consenting others.