The legality of lewd visual novels can be complex. In many places, if they contain excessive sexual or inappropriate content, they may violate obscenity laws. However, it also depends on the rating system and the context in which they are distributed. In general, it's a grey area and often not considered proper or acceptable in mainstream society.
You could try 'Jane Eyre'. It's a story of a young woman's journey through life, love and self - discovery. '1984' is also excellent as it presents a dystopian future where the government has extreme control. And 'Fahrenheit 451' which is about a society that bans books and a man's journey to question that.
You can also preview the novel if possible. Many e - book platforms allow you to read a sample of the book. By doing this, you can quickly get an idea of the writing style and content. If you notice any signs of inappropriate or lewd content in the sample, you can avoid the whole novel.
In general, they are not ethical. While the Renpy engine can be used to create a wide variety of visual novels, those with 'lewd' content cross the line. They promote an improper view of relationships and sexuality. There are so many other positive and creative ways to use the Renpy engine to create visual novels that are both entertaining and educational without relying on 'lewd' content.