Yeah, it was that last point that’s the sticking point. Like you I agree and would expect that when you post to a community you follow that communities guidelines, but since media apparently uploads to your instance not the instance hosting the community now you’re getting a 3rd party involved. If media was hosted on the same instance of the community I don’t think we’d have a problem and you wouldn’t need to worry about violating the posting guidelines of your home instance.
Personally, while I might create a “normal” account and a porn account, I definitely don’t want to feel like I have to do that. I think you should be able to create your account in any instance you want and post to any community you want (assuming said community isn’t on an instance which has had federation blocked from your home instance) without having to worry about the content rules of your home instance. I definitely think the embedded media handling needs to have a second look given to it.
You wouldn’t want instances to automatically store media posted from other servers, because that means that as a server admin you’ll have to deal with purging things like child porn from your server because some dickhead decided to cross post their trash to your community.
Wouldn’t you need to moderate the contents of the community anyway? Does it fundamentally change things whether the cross posting is done via posting from a different instance or by the poster just switching accounts? In either case the post is ending up in the community and you still need to moderate it. Having media stored in your home instance just makes things really awkward because now as a user I have to contend with both the server wide rules of my home instance and the rules of the community I’m posting in. I just worry we’re going to see things fracture into two nearly completely separate fediverses, there will be the NSFW adult fediverse, and then the SFW fediverse and you won’t be able to subscribe to content from one on the other.