There used to be a Kbin instance called feddit.online, which was shut down. @Jerry@hear-me.social just announced on Mastodon, that he brought feddit.online back to life, this time using PieFed. PieFed is a pretty neat alternative to Lemmy and Kbin/Mbin, created by @rimu@piefed.social and of course it’s fully free and open source on Codeberg: https://codeberg.org/rimu/pyfedi
It has some cool features like “Topics”, which are basically groups of multiple communities that you can view all at once (similar to these Lemmy feature requests: https://github.com/LemmyNet/lemmy/issues/3071 https://github.com/LemmyNet/lemmy-ui/issues/1113).
If this is so important to you, you are still very much free to start your own instance and see how far it goes.
The problem is not code. The problem is that no one wants to take this responsibility. Every one wants to talk about supportive they are on sex positivity until some men in uniform knocks on their doors because they are running a website that is available for minors all around the world.
Also, I don’t even want to get in the discussion of “sex positivity” being associated with “easily available porn”. Like you said, porn is easy to find and I really doubt that the someone who is savvy enough to use Lemmy would have trouble to know where it is.