Exactly, he is not hurting anyone if he does it... even daily. What's done online in the privacy of your own home stays online in the privacy of your own home. Though, that can be a slippery slope. First it's looking at 16 or 17 years old, 6 months later it may be 12 or 13 year olds... or even younger. U might want to give that a little thought.
I agree that a few questions arise. I'm wondering if maintaining such fantasies can indeed:
- make them more and more extreme and intrusive (as you said, the slippery slope argument)
- make you significantly more likely to commit a crime related to them
- make you hate yourself even more or create/aggravate a mental disorder, out of the sexual frustration or disgust in yourself
Having some proper insights on that would allow for more useful advices regarding OP's mental health, and I'd be very interested. Let me share my own experience and uneducated opinion about this: I've had very twisted fantasies since I was born. I did not particularly try to restrain them and it is true that they did get more and more extreme through time, but it's unclear whether trying to restrain them would have made the slope less or more slippery. Like OP, I often had this feeling of guilt and disgust in myself when I was turned on by new stuff, but this feeling often faded out with time and was replaced by self-acceptance and increased arousal/pleasure which improved my quality of life overall. It is true that, if I were socially competent, I could be more likely to experiment such things IRL (and for the most part it wouldn't even put me any legal trouble). *However*, I feel there's still a very clear distinction between the fantasy and what I'd be up to in reality. For instance, I fantasize a lot about non-consensual sex, but I could never ever ever ever ever enjoy such thing IRL because I'm not morally aligned with that at all. I also know I wouldn't put myself or someone else at risk of permanent injuries or death, although that's also part of my dreams. This moral boundary has never shifted in my mind, and I think that's what differentiates a simple "perverted weirdo" like me from an actual predator. IMHO, the fact that OP feels disgusted about the fantasy is a pretty good sign that his ethics is not ok with it, and therefore he would never be able to engage with an underaged.
So yeah, my hot take would be: as long as he's not accessing illegal material or abusing anyone IRL, he could be fantasizing about murdering a toddler that it'd be no one's business. If he feels that he'd be morally capable of commiting a crime to satisfy a sexual urge, then he probably needs to pause and get some help. But I'm not specially qualified to know where's the limit and I'd be very interested if someone had more data on this.