The court said the case should be retried to ensure the jury's conviction is supported by the latter argument.
The decision also urges the Legislature to examine the law, which was first written in response to cases in England that concluded fraudulent impersonation to have sex wasn't rape because the victim would consent, even if they were being tricked into thinking the perpetrator was their husband.
Willhite noted that the law has been applied inconsistently over the years in California.
In 2010, a similar law in Idaho prevented an unmarried woman from pressing rape charges after being tricked into sex with a stranger by her then-boyfriend.
The judge called what happened "despicable" but said the state's law left the court with no choice. Idaho's law was amended to cover all women in 2011.
Morales' attorney Edward Schulman declined comment when reached by phone Thursday.
Prior to the conviction, Schulman had argued Morales believed the sex was consensual because the victim responded to his kisses and caresses, according to the decision.