One thing I can't quite get is: why is porn a threat to Apple's security?

It's framed in the article like this 3rd-party app will compromise some aspect of Apple, but it's not really clear what. Don't iPhone users browse porn on Safari?

Brand image. Highfalutin brands like Apple don't overtly deal in vices.

Hah, and yet they are quite okay with gambling apps, either direct betting, or (especially) IAP-driven...

The parental controls works in safari and a parent can block explicit sites.

I guess that if a kid can now install an "apple approved" porn app, this is a threat to their business model of converting kids early to the Apple products ecosystem.

Parents can block alternative app stores. Furthermore, app age restrictions also apply to apps from alternative app stores.

https://support.apple.com/en-us/118128

Duplicate of https://news.ycombinator.com/item?id=42960219 ? DIfferent sources, but same event