You ever play Leisure Suit Larry? Pretty funny series, though not exactly A+ material. And guess what? ALL about sex.
Not to mention that the attraction of games is as widely varied as the attraction of film: You see some movies, say Bowling For Columbine, to get your thoughts moving. Others you see for the sheer style (Femme Fatale, maybe), or even just because you want to see a predictable genre flick (any one of the recent action/comedy/romantic comedy releases). Similarly, not all games have the same draws. I'd hardly call the draw of a story-driven game like Grim Fandango or Baldur's Gate 'mastering the system'.
It's not that sex has no place in games, it's just that publishers these days are too juvenile to incorporate sex into a game without making a huge deal about it. I recall a big brouhaha over Citizen Kabuto, about the Delphi character not wearing a top. Putting the top on didn't exactly detract from gameplay, but neither did having the top off. I, for one, would rather experience the product the way the designers intended it.