Is a presumptive skepticism of as yet unproven rape allegations immoral, anti
Singling out rape allegations for special skepticism would be problematic to say the least. As far as I know, there's no reason at all to believe that allegations of rape are less likely to be true than allegations of other sorts of criminal behavior. But in any case, skepticism and the presumption of innocence are two different concepts. The presumption of innocence is a legal principle about the burden of proof in criminal cases, and it has nothing to do with how likely it is in general that people accused of certain sorts of crimes are guilty. In the American legal system, before someone can be convicted, the state must provide specific evidence (not generalities about the kind of crime at issue) that establishes guilt "beyond a reasonable doubt." This means that even if a defendant is probably guilty, the probability might still not be high enough to meet the standard for conviction. A jury member might believe it's more likely than not that the defendant committed the crime and yet might not vote to convict because "more likely than not" isn't the same as "beyond a reasonable doubt."
A juror who takes the "presumption of innocence" seriously will be on the lookout for weaknesses in the State's case. We might use the phrase "presumptive skepticism" for that way of approaching the evidence. But this has nothing special to do with rape and it has nothing to do with how likely it is in general that people accused of rape (or any other crime) are guilty.