NSFW is already off by default when you sign up to most instances
I don't think this works in practice. Most big instances have gone the extra step to also defederate from the two major porn instances here. Showcasing that there are additional issues, otherwise they'd just have used this instead. I took a quick random sample of the biggest Lemmy instances and ~50-60% additionally block them entirely.
NSFW and 18+ aren't the same thing. The NSFW tag is made for a slightly different purpose. And it's a crutch that doesn't work well for this purpose. There are some slightly vulgar topics that shouldn't inadvertently pop up at your workplace but they might be safe to consume for minors. Also I think minors should have access to sex education. The Wikipedia has a similar stance. There are videos of "the act" on Wikimedia. You shouldn't watch them while sitting in your open-plan office. But I think especially with the situation of sex ed in the USA, adolescents should get a chance to ask their questions and learn something about important aspects of life. The NSFW tag as is is doing them a disservice because now they can't. Or everything else immediately gets mixed in. For example I'm not comfortable sharing my experiences >!sticking bluetooth-enabled things into somebody's bum!< with kids. Or having sex ed and hardcore fetish stuff being the same category.
And I mean it's not even just that. Gore and pictures of dead bodies in the Ukraine war also fall into the same category. So everyone just gets a yes/no decision on everything ranging from sex education to gore. In practice both these extremes aren't very common on Lemmy. But in theory it's just like that. (It's not entirely theoretical. We just have a different community here. But there are examples in the wild. For example 4chan mixes pretty tame porn with fetish with crime, gore and death.)
So in summary the current state of (mis)using the NSFW tag actively leads to defederation and it's doing a disservice to both people who participate in adult conversations and also adolescents. And its overly simplistic design prevents some conversations which should be allowed.
Why do you think porn is bad/unsuitable for 14-17 year olds?
My opinion doesn't really count here. There are legal requirements people need to implement, whether they like it or not. So that's kind of already the end of this conversation.
I think it makes a difference if you watch (somewhat tasteful) plain sex, or >!somebody dangling from a hook in the ceiling getting whipped by a disguised old man!<. I think it's just not the same category. And we shouldn't treat it as such. Similarly it's also not the same if you deliberately explore that when you're 17. Or you're inadvertently exposed to it when 12 while researching what sex education has failed to provide you with.
And there's the aspect of me inviting friends and family to my self-hosted services. Or discussing Linux server administration with people. I don't want to mix either of that with porn. I think having it hidden per default is a good first step. And just requiring an extra, deliberate step to enable it is a good design. It just lacks any of the nuances to it, mingles valid use-cases with filtering that is made to do something else, and as I pointed out with the defederation happening, it comes with issues in practice.
And I think the Fediverse offers some important advantages over other platforms. ActivityPub is very vague. We can just attach fields to label content and the technical aspect is kind of simple to implement. And with federation, we have diversity built-in to the platform. This is our unique advantage. People have different use-cases, different moderation needs and perspectives and opinions on something like my proposal with the filtering. And I think the Fediverse turns out to be made exactly for something like this. I could have my instance my way and someone else can have a different opinion and have their instance another way.
But it requires some coordination effort. We need to agree on a foundation and some technical aspects. I don't think a crazy rag rug works as a whole. And we already see some consequences of other disputes. Moderation being a constant issue in the background and instances seperating from each other because there's no nuance to moderation and defederation. And ultimately we want to talk to each other and connect. And provide everyone with a place they like.