So, they're not blocking only porn. They're blocking a wide range of sites with various reasons - for example: selling illegal drugs (including mental, abortion drugs), copyrighted sites (torrent, etc), praise about north korea, etc...

When they've started to terminate TLS, the reason was to terminate illegally shared webtoon (web cartoon) sites.

For more info: https://en.wikipedia.org/wiki/Internet_censorship_in_South_K...