I think it is a mistake to draw generalities.
Something sexual clearly took place. But the people responsible for proving beyond a reasonable doubt that the act was not consensual cearly did not believe that they could do so.
Two important principles come into play here. First is the presumption of innocence. The second, equally important, is that the role of a public prosecutor is not to secure convictions, but rather to secure the administration of justice. Prosecuting a shaky case is not fulfilling a prosecutor's duty, but rather a violation of it.
But that does not mean that events did not unfold as M. Strauss-Kahn's accuser has alleged.
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--James