It is not often that a judge presiding over a criminal case will accept "I just didn't know," or "I didn't understand" as a valid reason to uphold an acquittal or to reduce a generally accepted sentencing term. This may be especially true in cases of alleged sex crimes, given that many convictions carry mandatory sentencing terms and one cannot generally argue ignorance with respect to the elements of a sex crime.
However, one recent holding by the Wisconsin Supreme Court suggests that with respect to sentencing in particular, "I didn't know" may be a legitimate enough argument to warrant reduced sentencing.
The case before the court involved a man who was convicted of sexual assault of a minor under the age of 13. He was then sentenced to a mandatory minimum sentence of 25 years imprisonment.
The due process problem occurred when the man was not informed about the mandatory minimum sentence during the plea bargaining process. Had the man known that a conviction would be impacted by a mandatory minimum, he may have chosen to accept a plea deal before he was convicted.
The Wisconsin Supreme Court ultimately held that it was a violation of state law for prosecutors to fail to inform the defendant of all possible criminal penalties that could be associated with a conviction. As a result, the man's case has been remanded to a lower court in order to determine whether his case was prejudiced by this alleged disclosure violation.
Though one cannot generally argue ignorance in a sex crimes case, due process violations related to the ignorance of the defendant may be significant enough to warrant reduced sentencing in Wisconsin.
Source: State Bar of Wisconsin, "Failure to Inform Defendant of Mandatory Sentence Could Be Prejudicial," Joe Forward, Aug. 9, 2012