A dating sim is mostly about number crunching and setting off flags for a specific girl. The point is to maneuver through a maze of logic choices, get the right flags, avoid the bad flags, and eventually your score gets tallied. This is mostly just a mechanic. Whether the story around it is this or that is solely dependent upon the author and publisher. A game like Galaxy Angels uses dating sim mechanics but the gameplay is mostly star fighter esque and visual novel reading.
An eroge is simply, at its base, 18+ rated. Many are written in the visual novel formats. Such as Tsukihime. Some are hybrid SRPGs like Eien no Aselia or Utawarerumono. Actually, I heard Tsukihime was originally made as a doujinshi: fan based production. [This is like your friend making a starter company out of his garage)
When an eroge is sold in the mainstream culture, which means PS1-3 and PSP consoles, most of the time the mature content is removed. So you could say the production is meant to be dual use in terms of appealing to two different demographics: Between kids and young adults then actual adults.
I say “most of the time” because depending on the content, removing just the sex won’t remove all the mature content. Short VNs like Saya no Uta probably will never get an all ages rating even if you removed the sexual parts. The mature parts are integral to the story and it would greatly exceed in changing the original story past recognition. Only manga and anime adaptations practice dramatic changes so routinely.
As for the overlap between sims, eroge, and visual novels, it all goes back to their definitions. A sim has a simulation component. An eroge has an 18+ sexual content rating. A visual novel has text you read and a choice you make sooner or later. Clannad is one of the longer VNs around. It was originally created for all ages, unlike Kanon and AIR, which was first released for 18 plus audiences. When a title has mature content with the 18+ rating, it is an eroge. If it also has simulation number crunching, then you could say it has dating sim mechanics. And if that’s the only distinguishing attribute (number crunching), then it’s a dating sim. If it has a text story and plot, then it could be both an eroge and a VN or an eroge and a dating sim.
Usually though, you don’t find dating sims and VNs to be compatible. Dating sims usually require more user input, like a real game, which puts it closer to SRPGs than VNs. A visual novel is just that. It reads and plays like a novel. You flip pages. That’s about it when it comes to user choice. When you finish the book, you then reread the book, except this time you might have “different endings and choices”. That distinguishes it from a paperback novel, plus Visual Novels combine movies, audio, and text together in one cohesive design.
Eien no Aselia is a hybrid VN and S(story/fight simulation)RPG. Utawarerumono is a hybrid VN and S(strategic/tactical)RPG.
They usually conform to their gaming production company’s history. Games like Daibanchou are sort of like an eroge, SRPG, dating sim all combined. It’s a jumble of a lot of different mechanics. But they’re not visual novels because the gameplay isn’t that of a visual novel. A VN is pretty specific about how you play it, so there’s less variation around. Air and Kanon, on the other hand, are visual novels. And since they don’t have number crunching, there’s no way you can say they have dating sim elements.
Planetarian is neither a dating sim, an eroge, or an SRPG. It is, however, a short VN, aka kinetic novel.
FSN (Fate Stay Night) is an eroge/VN. Ore no Imouto had a satire CD promo image around episode 9/10 on Kate Stay Night: Unlicensed Blade Works. A good bit of fun concerning Unlimited Blade Works, the 2nd of the 3 total routes for FSN the pc game rated 18+. For things like FSN, the eroge content is pretty low compared to Utawarerumono or Tears to Teara. So for matters of classification, it is generally more useful to simply label it as a VN with an age rating.
Dating sims, VNs, or eroge are not “genres”. They are not fantasy. They are not science fiction. Nothing labeled a dating sim, a VN, or an eroge will ever tell you what the content is about. It’s more of a production format.
Hard pornographic production companies or games will always cater to the more extreme range of the adult sexual market. It’s the same for porn in America as well. Since there is no/little (story) content there, you don’t have to worry about it. It’s not the case in Japan given how their censorship laws work.
As for “hentai”, that’s the point. Hentai is porn. Specifically, strange or BDSM hardcore porn. It’d be like going to the BDSM section of hard core porn and being surprised there’s leather and whips in it as “keywords”.
Anime, meaning mainstream broadcast on Tokyo TV anime, has little relationship with hentai. Hentai, in fact, is more related to adult material, aka pornographic magazines and videos. They serve the same niche even though one’s made by drawings and the others’ made with real actors.
Btw, Chaos Head and Himawari Shoujo Sharin no kuni are two other titles I thought I would mention. Both are visual novels. Although the first is only a satire about eroge while the second one actually has sexual scenes in it. But otherwise, the story doesn’t really change if you remove the H (ecchi) scenes due to the fact that the story in those visual novels are the “Primary” draw.
This post has been edited and hopefully it will be of greater help to those taking a step on this strange road called Japanese entertainment media.