Sodomy is still illegal in many places. Including some states in the US.
What about drug use? Illegal, but still acceptable despite that? Why, why not?
To some people, any bodily fluid or excretion is literally just as disgusting as scat.
Vore has strong overtones of murder, rape, and cannibalism built into it, all extremely illegal(not to mention pretty universally morally wrong). You can exclude all those things if you want but the core idea is still rooted in them. Still more acceptable than babyfur?
Conversely, many babyfurs avoid nsfw art like the plague because they're not fucking pedos. Quite a few either don't wear diapers for real or if they do wouldn't soil one if you paid them. And ageplay, between two consenting adults? So far as I know there's no laws anywhere against that. I'd be a little surprised if there was.
Hell, master/slave relationship and such. Slavery is illegal. Where's the line? Is it fine cuz someone says they agreed to it? what if they're under coercion or they've been conditioned? what if someone's fantasy is to SAY that's the case when really they're there 100 voluntarily and they'd kill the fantasy and leave in a second flat if their dom did something they shouldn't?
Funny things about lines in the sand, sand isn't very stable. It moves around with the wind. Lines fade. New lines can appear on their own naturally. Not a very solid place to draw a lasting line.