There was a pretty damn good video on this topic that Kotaku posted about not too long ago...
http://kotaku.com/5011043/video-game-sex-lecture-time
(Edit: Kotaku link wasnt working, this one should)
http://www.rockpapershotgun.com/?p=1810
The guy uses a "Zero Punctuation inspired" format to get his point across. In the end, its both a very entertaining and very well informed opinion. The crux of his argument is basically that video games are a young artform and need more time to develop mature and complex sexual themes. Once it becomes a more respected (by mainstream society) artform, this change should come naturally. Not only will society start accepting more sexual content in games (as in TV and Movies), but also games themselves will develop a more sophisticated take on the topic. What this boils down to is essentially less gratutitous and unnecessary cleavage (Tomb Raider), and more tasteful and intricate story/gameplay that involves sex (Mass Effect).
