Actually, this isn't all that new. Many ero-games did become animated. However, they were only restriced to being "18 and older" pornographic OVA anime releases. It is simple to say that "well duh, the original game was erotic, isn't it obvious that the anime is going to be porno material for the adult video market?" However, the truth was that the anime versions of these ero-games barely touched on the actual story and plot lines - most of the animated parts were focused on sex scenes. In a sense, these OVAs were not truly "the anime version of the original ero-game."
But then, one anime began to open a path to change all that. In 1998, an anime called "Night Walker ~Midnight Detective" was aired as a twelve episode anime series (obviously sex scenes were cut out). The original ero-game was released back in 1993. This is the first actual anime TV series that was based on an ero-game. Around the same time, "Dokyusei 2" also aired, but this was more like "re-hashing the erotic OVA episodes, and editing them without the sex scenes for airing on TV."
While it "Night Walker" and "Dokyusei 2" did get attention at the time, the anime itself didn't do so well.
However, in the same year, the TV series "Sentimental Journey" which was based on the gal-game "Sentimental Graffitti" did pretty well (pretty ironic since the original game was crap). The success of this ignited the light that "TV anime inspired by ero/gal-games actually had some market value"
The first real successful TV anime that was inspired by an ero-game was technically AQUAPLUS' (the consumer arm of the softhouse Leaf) "To Heart" in April of 1999.
By this time, the Japanese anime industry was in a situation where dozens or so anime companies were scrambling against each other for short 13~26 episode late-night spots on TV channels. Otakus call this "shinya-waku ranritsu jidai" (The war of late night TV spots).
Anime companies wanted ideas fast to get a lead from their competitiors. Ero-game companies yearned to "make something like an anime, but don't have the money to do so." Eventually, the two got their points together, and you started have more and more anime based on ero/gal-games being released.
Just from what I can think of, here are the "anime based on ero/gal-games as they increased by year":
Night Walker ~Midnight Detective~
Kakyusei ~Anata dake o Mitsumete~
Sakura Taisen ~the Movie~
Pia Carrot e Youkoso! the Movie ~ Sayaka no Koi Monogatari~
D.C. ~da capo~
Yami to Boushi to Hon no Tabibito
Kimi ga Nozomu Eien
Kita he ~Diamond Dust Drops~
Tsuki wa Higashi ni, Hi wa Nishi ni ~ Operation Sanctuary~
Wind -a breath of Heart-
To Heart ~ Remember my Memories~
Kakyusei 2 ~Hitomi no Nakano Shoujo Tachi~
Mahou Shoujo Lyrical Nanoha
Harukanaru Toki no Nakade
2005 [so far]
AIR (TV series)
AIR (the movie)
Suki na Mono wa Suki dakara Shouganai
Lime-iro Senkitan X
As you can see, more and more big-hit ero-games started coming onto the TV anime format. Leaf's "Comic Party," CIRCUS' "D.C. ~da capo~," Key's "Kanon," and age's "Kimi ga Nozomu Eien," just to name a few.
Then again, there are also anime based on ero-games that didn't do so modestly - such as "Popotan" and "Yami to Hon no Tabibito." These were more like they were selected due to the high quality of their CG work (both of these ero-games were done by pretty famous illustrators). In the end, even a doujin game - TYPE-MOON's "Tsukihime" went on to become an anime. Right now, it is not so modest to say that currently, "ero-games are created with an anime market in their view." I mean I'm sure I am not the only one who already sees "Fate/stay night" as an obvious anime marketing material.
In 2001, the first movie based on a all-ages love simulation game was released - Sakura Taisen The Movie. In 2002, "Pia Carrot ~Sayaka~" became the first anime movie that was based on an ero-game. Early next year, the second anime movie based on an ero-game, "AIR" will be released. It is astounding that the director for this is Sir Dezaki Osamu - a highly respected veteran anime director who directed major classic anime hits such as "Kyojin no Hoshi" and "Ace o Nerae!" In 2005, the first TV anime based on a ero-bishounen game (ero-game targeted for girls), "Suki na Mono wa Suki dakara Shouganai" will also begin airing on TV. The momentum for ero-game turned anime is unstoppable.
Non-erotic bishoujo games turned anime also increased. As noted above, in addition to "Sentimental Journey" (based on "Sentimental Graffitti"), you have "Kita e ~Diamond Dust Drops~," "Sakura Taisen," and "Harukanaru Toki no Nakade."
Perhaps, you can also add animes such as "Sister Princess" and "HAPPY LESSON" to the list. I mean, "you suddenly have 12 young sister all in love with you," or "you have five beautiful moms as your teachers looking after you." The plot line screams that it could've come from an ero-game. But, I intentionally left these two out since these two were original ideas that were serialized on Dengeki G's Magazine.
Then what kind of anime do these ero/gal game based stories evolve into? Well, duh. If you take in what was in the original story as an anime, you have a single male lead revolving around dozens of pretty girls - a typical harlem anime. While going through each sub-heroines' story, the main plot line evolves by maintaining and growing the relationship between the main male lead and the main heroine. Then there are those anime where no male lead exists at all and everything is told from the female characters' point of view ("Popotan" and "Yami to Boushi to Hon no Tabibito"). And then, there are those anime where each episode focuses on each individual female characters without the existence of a male lead ("Kita e" and "Sentimental Journey")
And then there are some interesting titles such as the anime version of "Comic Party." The main story of the anime involved the male lead, Sendou Kazuki working hard to attain the pinnacle of the doujin world. Interestingly, this ero-game based anime had the female characters as supporting roles rather than a love interest.
"Mahou Shoujo Lilikal Nanoha," which currently started airing this season is also pretty interesting to note about. This all began as a spin-off of the ero-game "Triangle Heart 3 ~ Sweet Songs Forever~" as a joke. The extra joke story within the original game was "what if we take the main character's younger sister and make it into a magical girl story?" (Think of it as something like the relationship of "Tenchi Muyo" to "Pretty Sammy" or "SoulTaker" and "Nurse Witch Komugi-chan"). Somehow this joke began to turn into reality (which is why the saying goes in Japanese anime industry "don't make a joke without thinking about the consequences"). What you have here now is a mahou shojo story that was originally developed as a joke plot within an ero-game.
Another interesting note is on the voices for these types of ero-game turned anime (or ero-game turned consumer console games). There are usually two ways these occur:
The seiyuu cast is entirely different from the ero-game and the anime/console version ("D.C. ~da capo~")
The seiyuus sound exactly the same, but the names are different
Focusing on "B," you have a character on the ero-game version and a character in the anime whose seiyuu's names are different, but they sound exactly the same. This is because the seiyuus use pseudonyms (even they themselves don't mind, their talent agency does not want the image of their seiyuus doing the voices for an erotic game, especially if they are rather well known seiyuus). Hence, what you have is something written like "the reason why you think they sound the same is because the seiyuus are long-lost relatives to each other" (How it was explained on NekoNeko Soft's HP for the consumer versus ero-game version for their game, "Mizuiro")
Say for instance, you have a big name female seiyuu who has leads roles in NHK children's anime show, using a pseudonym when she plays the role of a nice and grudging sister on a big hit ero-game (more bluntly: Mizuhashi Kaori = Uehara Tomomi). And you have a big name male seiyuu who in the light plays the role of a hard-broiled car driver while he uses a different name to make appearances in many ero-games. (Once again, more bluntly: Koyasu Takehito = Jumonji Hayato)