What is the test for proving beyond a reasonable doubt in a sexual assault case?

"It is a fundamental principle of our criminal justice system that the prosecution must prove beyond a reasonable doubt `every fact necessary to constitute the crime with which ... [the defendant] is charged.' [Citation.] It is also clear where evidence presented to the jury indicates more than one act which may constitute the charged offense, the jury must be instructed it must unanimously agree on the specific criminal act in order to convict the defendant." (People v. Hefner (1981) 127 Cal.App.3d 88, 96.)