I think the person above you is referring to CP, which I think is a very different harm than you’re talking about.
For what you’re talking about, I’ve heard a couple of explanations.
The first is one you pointed out, a largely religious objection to non-traditional sex. They find any sort of fetish immoral and worry porn will push children into fetishes as adults.
The second one you missed and I find reasonable, I just don’t see porn bans as the solution.
The theory is that porn will make children aware of more “extreme” or “unusual” varieties of sex, but it doesn’t endow them with the ability to have an open dialogue with a partner about wanting to do that, whether their partner is comfortable with it, etc.
The fear is that young adults will accidentally pressure their partner into sexual acts their partner isn’t comfortable with. They either see it often enough on PornHub they overestimate how normalized actually doing it is, or they’re young enough that discussing sex with their partner is still uncomfortable.
To be clear, this isn’t a puritan stance. It’s not saying that eg BDSM is bad, but that participating in BDSM should be prefaced by a conversation about safe words, boundaries, likes, dislikes, etc. Without the context of having been sexually active, it’s hard to know what parts of PornHub are “normal” and implicitly agreed upon by agreeing to have sex, and what parts really need a separate conversation.
To me the solution is mostly sex ed, though. Open communication solves most of this. The only ones I think would still be a bad idea mostly hinge around control. Eg CNC is probably a bad idea because the lack of control is part of the appeal, but it could also cause someone inexperienced to not speak up and say they aren’t enjoying it.
Maybe I underestimate the younger folks and times have changed, though.