Most adult video games I've seen are either extremely bad porn, or mildly raunchy games. I'm not an expert on the subject by any means, but here's my take on the situation:
There's the user-made mods for normal games, but they are generally just nudity and not so much porn. Tomb Raider is, of course, a classic there.
Then there's the "adult themed" games, such as Leisure Suit Larry. The first games were generally more humor than softcore porn, the peak being a few cartoon breasts at worst. The very last one is a bit of humor, a lot more smut, and a whole lot worse (fairly clever problem solving replaced with inane minigames. Ugh). The Lula series is another example, again with a fair bit of humor. Apparently there's supposed to be a chess game or two with sex in it as well.
Then there's the virtual sex-type games, like Sex Villa (think that was the name) that's basically a fairly limited interactive porn movie with 3D characters. I haven't tried any of them, but they don't look too impressive.
Finally, the whole hentai game market. But really, they are awful on all fronts unless you're into seeing manga characters doing things to each other that would make even Max Hardcore re-evaluate the morals of the sex industry. If you have no particular problem with horrendous dialog about how your character enjoys raping his mother while forcing her to urinate on a portrait of his dead father (that is not a joke by the way, it's an actual scene from a game), I guess you can give them a shot. Personally, how any normal human being could find them even remotely erotic is beyond me.
Frankly, if you want porn, video games is probably not the best place to look. The chances of finding a quality game that is actually erotic are slim to none as far as I can tell. Let me know if you find a half-decent one, it'd be fun to be able to recommend at least one game if someone asks in the future.