Japanese games traditionally take their thematic (and moral?) cues from anime/manga where "ecchi" style characters and scenarios are commonplace.
The Western developers too are only taking their influences from TV/Cinema, albeit the less sophisticated side (although we could get into a art imitating life debate on that one).
Personally I think the "problem", if it is one, is that games don't really handle narrative content all that well unless it's built into the interaction. To create a deep and complex narrative that would be required to explore the subject matter of sexuality is (currently) beyond (mainstream) gaming.
Also the core audience (which is the same as that of action films, modern teen-slasher movies, fratboy comedies etc.) know what they like and like what they know and buxom ladies with 'tude is what they get and they lap it up.