Many people don't really want great sex. Good sexual experiences can be emotionally overpowering-- mind-blowing, rather than warm and comforting. Lusty sex requires you to confront all kinds of worriesss getting so close to your partner that he or she overwhelms you, or being rejected at an intensely vulnerable moment. It may even put you in touch with your own mortality, reminding you that your partner won't always be around. Great sex requires inner reserves to tolerate the angst.
Sigh And it happens so rarely, too.
A pretty good article, even if I don't buy all of it wholesale.