Just like how no liberal can answer "What is a woman?". No liberal can answer "Where is the line drawn in regards to kinks?" either. That's interesting.
1) Whatever doesn't cause serious harm (physical or psychological)
2) Whatever doesn't impede (non-consensually) the freedom of another
The main point of contention is 1) even though they're arguing from the perspective of upholding 2)
One side believes in restrictions in order to reduce harm
The other believe that freedoms must be maintained even if harm is a factor
From my understanding the lines are:
1) Consent (both parties must
want it, and are capable of giving it
)*
2) It mustn't cause unwanted harm to another person.
Theoretically there is no line so long as it abides by the above. Scat, ageplay, etc—all permissible.
The issue here is this is a universal principle that may or may not be applied in general by the person.
It becomes contentious when applied to kinks and porn, but when applied to guns, speech, alcohol/cigarettes, etcetera, you might find more people in alignment with the more Liberal POV. If getting rid of gross porn and gross people also meant restricting some of their own freedoms in doing something non-porn related, they might side with the "sicko" over the "non-sicko".
I particularly hate the inability to concede that there is SOME connection between pedophilia and what is tantamount to simulating pedophilia. It's mendacious. I can't buy this guy both being so well-versed in ABDL to defend it like this, but then also naive to the "caretaker" role in the ABDL dynamic and how it's more or less someone who wants to pretend to partake in child molestation with an adult surrogate. It's possible for someone to have a coherent defense for it as a paraphilia without playing coy about the sinister side of these "adult babies" and the fantasies they're enacting.
At this point, he's only dying on the hill in the sense that he's rhetorically blown his legs off and refusing any assistance to not die on the hill. We all know that if we sat through an hour of "ABDL kink content" that it'd be pretty damn obvious pedophilia plays a role in what grinds their gears and excites them. Then the goal posts move again to "that's only some of them!" and then the goal posts just keep moving forever. It's really that Spongebob meme of Patrick refusing to accept that it's his wallet.
Because it's become important to them. It's a major source of happiness/dopamine and so they won't see it tarnished.
I personally don't see why there's an all or nothing approach when there's obviously nuanced involved, regardless of your view. Obviously it's
possible there might be some paedophilic element involved, but it's just one of a laundry list of possible reasons. His problem is refusing to accept it, but the problem being raised by opponents is that it's
only paedophilia.
From his POV accepting conceding to that possibility would effectively open the door to labelling him a paedophile, which he likely isn't considering he saw the kink as "pure"/"acceptable" enough to put front and centre in the OP post. From the bizarre number of pedos who have outed themselves as pedos on this very site, they usually come right out and say it, because it pleasures them on some level.

I don't think he's a pedo and if he's into ABDL, his reasons are likely not tied to paedophilia. Like many kinks though the true underlying motives are still likely to be embarrassing (which he may enjoy—though not to the extent that he's regarded as a pedo) such as: envy a child/envy his past-self (re-experiencing a time in his life he wouldn't have the memories to appreciate in retrospect), like the idea of being infantilised, being doted on by a "mother"-like figure, being doted on by a proxy for his
actual mother, get off on the humiliation, etc.
TLDR: It
could have a pedo-basis, but it could
also be any number of things. He probably doesn't want to be labelled as a pedo—a battle he knows he'll lose because to some people denial is proof of the accusation made—but one of his paraphilias has been disparaged and so he needs to defend it, or not be lumped in with one of the "bad ones".