I seriously doubt that visual novels with sexual content are the majority. For every porn VN you find on itch, you will find multiple short story VNs with no sexual content whatsoever. The porn just tends to rise to the top because (1) lots of people like porn, (2) it's easy to demonstrate the quality of porn through screenshots, and (3) it's a lot easier to write serviceable porn than it is to write an actual non-porn story that's worth reading, because there are so many non-porn stories to choose from.