Recently, with the game series Dragon Age (Origins and II), Mass Effect (1 and 2), and even Grand Theft Auto IV, there has been a focus on relationship management in-game. GTA IV was a bit heavy in this respect, whereas you can ignore it somewhat in the others if you want. But, the thing is, if you choose the right dialog options with the right characters, you can have sex with them. Ok it isn't usually shown very explicitly, isn't realistic (nor is the dialog or the whole scenario in the first place), and it seems like an odd addition to the game that isn't related to what the point of the game is.
|An LSL screenshot - love me some pixels.|
But there was a game series where sex was the entire point of the game, the Leisure Suit Larry series (which, according to Wikipedia, survivied longer than I thought). I remember them from when I was a teenager, although I never played one. They were generally thought of as rather weird, they weren't quite games, they weren't quite legit uses of your computer, and they weren't real porn either (now we have the internet for all that).
But with the intentional, direct inclusion of dialog to get to sex in hugely popular, successful, and mainstream games, Larry is vindicated to some extent. (I still don't find these weird sex scenes in games anything worthwhile or even hot, they're just creepy.)
Scenes are easy enough to find in YouTube. Here's a search for Mass Effect 2 scenes, and a video making fun of one (a combination of the gameplay footage and real people -- that makes it sound creepy -- real people as if they were other characters on the ship who do not get it on with the digital in-game characters). I've tried to watch a few to see what they are like but they are all so creepy right away I give up. (And I gave up on Mass Effect 2 since I found it rather tedious and repetitive.)
Edit: I totally forgot The Witcher, which I really see no reason to play.