How can such a fundamental element of thr human experience can be so conspicuously absent for almost all art and media.
What are you talking about? Heaps of movies have sex scenes. Heaps of songs are about sex. There are heaps of books and other stories about sex. The internet is packed with sex stuff of all kinds. Advertisements in the street are obvious implicitly or explicitly about sex. So how can you say that sex is 'conspicuously absent from almost all art and media'? Are you looking?
Allowing explicit porn on twitter doesn't make it ground-breaking in any way. It just changes the tone and target audience of the site, such that you will now see porn inserted into basically any conversation or topic.