After watching/reading anime and manga, I recently started going into visual novels. One thing that struck me was the fact that erotic content (aka H-scenes) seems to be quite common in the world of visual novels. Examples: Fate/stay night, Muv-Luv Alternative, G-senjou no Maou. Can anyone explain this higher prevalence of erotic content in visual novels, as compared to anime and manga?
Because visual novels are spinoff of eroge games. According to wikipedia:
The eroge game (or H-game, hentai game) are introduced into the market (no pun intended) in the 1980's as a means to attract users to the (game maker) company computer platform.
In 1992 the dating sim / love simulators were born, when a company designed the game so the player had to win the affection of the character before gaining access to the H-scenes.
The first use of the term visual novel is in 1996, and from there the erotic material became optional, with the introduction of the all ages versions of the visual novels.
But still adhering to tradition, the visual novels of today still have the H content, though not every one.
Refer to the wikipedia article to more in-depth historical facts, including game titles and company names.
I upvoted @Mindwin's answer because the answer's content is, to my knowledge, absolutely correct.
But to clarify a little more: in the period 1980–1996 or so, the kinds of games that we'd call visual novels nowadays were a lot more like hentai anime. The stories were usually fairly simple and pretty much revolved around a character talking girls into sleeping with him.
But the different types of games mentioned in Mindwin's answer started to converge on each other. My impression, and the Wikipedia page implies the same thing, is that the first eroge which prioritized story over sex was Kanon. In Kanon, the sex scenes can be totally excised from the story without much loss; they're essentially what the authors of How Not to Write a Novel call "benevolent tumors". Pretty soon, other games, such as Type Moon's Tsukihime and Fate/Stay Night, did the same thing and made a game with a deep and interesting story that only incidentally contained sex scenes.
Now we're at the point where some prominent visual novels, such as Clannad, have no erotic content at all. Others, such as Little Busters, are released without erotic content and later re-released with erotic content added in. The modern visual novel is sort of an evolutionary crossover between the original eroge, which always had eroticism, and the renai games like Tokimeki Memorial that usually didn't. Its lineage contains games with erotic content, so visual novels are more accepting of erotic content than more mainstream forms like anime and manga. (As Memor-X points out, this is also somewhat inherent to the nature of the media; anime are broadcast on TV and manga are sold in bookstores, so it's harder to sell anime and manga with sexual content unless they have no other purpose for existing.) But because the sexual content of visual novels is typically easy to remove, and because the stories are often unique and compelling, it's common to simply remove the sex scenes and adapt them to anime and manga.