More instruction on red flags in anime.
Harem: a show in which a lead male is surrounded by female characters
Examples: Kanon, Clannad, Love Hina, Happy Lesson, Tenchi Muyo, Seto no Hanayome
Examples: Yamato Nadeshiko Shichi Henge, Ouran High School Host Club, Fruits Basket
Probably one of the top 5 most popular genres. Its popularity may have to do with the ocean of dating sim games in Japan in which the object of the game is to make it to bed with one of the many girls characters. When we use the word Harem in anime, it doesn't always imply a suggestive situation. Certainly there are shows like Saishuu Shiken Kujira that take after the girl games mentioned above. The definition of Harem used to mean that the girl characters were trying to end up with the (indesisive) male (ex. Love Hina.) But with more recent usage, fans call shows which merely feature many girls as Harem.
According to Wikipedia, these shows are a type of Hentai that involve blood, gore, disfiguration, mutilation and body fluids. Not all shows that have pornographic, bloody scenes are ero guro. It is an artistic genre. This is the genre from which "tentacle rape" emerged. Because of the perverse nature, this genre is tucked away with Hentai and it not very available or popular among fans - praise God!