You know, you can run your own DNS which points at that one, and implement additional rules. That seems like less work than just using your ISP's DNS and needing to implement more rules.
Reddit isn't a porn site in the sense that if you go to "reddit.com", you're automatically shown porn. It's a diverse site that has adult content areas.
Reddit has profile settings like:
[ ] NSFW content hide images for NSFW/18+ content (Don't show thumbnails or media previews for anything labeled NSFW)
[ ] I am over eighteen years old and willing to view adult content (required to view some subreddits)
Needless to say, these are implicitly off or a visitor to the site who does not have an account.
Still, Reddit will not prevent visitors from seeing "adult content" in a more general sense:being exposed to adult discussions that can touch on any sort of topic from any angle.
If we're talking about protecting kids, I think a whitelist of allowed sites will be better than a blacklist, which is intractable.