Statistically speaking, mediocre sex is far more common than spectacular sex - and therefore far more likely to happen to you. You'll be pumping away, only to look up and find your lady staring at the ceiling. Depressingly, although women may rank bad sex alongside sipping their own vomit, most won't try to bring you up to speed. Instead they'll simply whine on to their mates, inadvertently spreading a vicious circle of shame that will prove disastrous for you in the future.
http://www.fhm.com/bionic/bettersex/bettersex.asp?id=1