What is the test for overturning a guilty plea in a sexual assault case?

2 In Cotner v. Henry, 7 Cir., 394 F.2d 873, the defendant, as a result of his wife's complaint, had been charged with sodomy committed upon her. He pleaded guilty without having been advised there was a substantial constitutional question whether the statute could be enforced against the partner to a marriage in the privacy of the bedroom, in the absence of force. The record did not show whether force was used. Habeas corpus was held available to set aside the plea for the purpose of a remand. The court observed: