I don’t think he decided to end it all over AI
It's hard to know what's going on in someone's life. That is, the proximate cause of him storming off might have been that argument, but that didn't mean that he didn't have other things creating pressure.

checks
https://lemmyverse.net/communities has a list.
It's on the lemmy.porn instance, !forcedincest@lemmy.porn. Presumably that instance is, well, for porn.
If you don't want to see it, you can block that community and you won't see any more from it. If you don't want to see anything from that instance, you can block that instance. If you don't want to see NSFW content in general, you can block that.
As to "why" in the broader sense, it's because the Threadiverse is a global system. It spans many countries and different groups of people. It's like asking why something is "allowed" on the Web
everything is allowed as long as the local country is okay with it.