I think people on the whole are probably moderately less offended by low-resolution nude Sims skins than by a minigame in which your goal is to bring the pleasure meter to a climax by tapping a pair of keys to control how you slide it in and out.
Ah, I didn't realize this was actually interactive. Still, even that has been done before; I remember hearing about a fan game made using the really old sierra adventure game engine (the 16 colour text-based one used in Police Quest 1, Space Quest 1, and of course Leisure Suit Larry 1) called Sex Quest, where you goal is to "score" with a certain woman. Once that's done, you get to play out your "reward" via a mini-game... though of course the blocky EGA graphics would mean it's far from realistic.
Since GTA:SA is the third?
Fifth, though for some reason the developers decided that neither Vice City or San Andreas are "Grand Theft Auto 4"; presumably when GTA 4 actually does come along, they're aiming to revolutionize the series or something.
The reason you (as well as many other people) might have heard there are only three games in the series is because the first two GTAs used 2D graphics and were cult (as in, not many bought them, but those who did loved them) PC, PS1 and Gameboy games. When the series went 3D and from PS1->PS2, it suddenly got ultra popular.