In many US states (and in the UK), statutory rape is a strict liability offense. This means that there is no intent requirement at all; the only allowable defenses are those that the elements ofnegate the crime are not presentactual act (i.e. therethere was no sex or, the victimperson was over the age of consent)age, or sometimes that the action was not a conscious or voluntary action), it falls within a statutory exception to the crime, or there is an applicable defense appliesthat has nothing to do with intent. Many general defenses do not apply to strict liability crimes; in particular, "I thought X when Y was true" tries to show there was to intent to commit the crime, which is irrelevant.
In Michigan (where the crime took place), statutory rape is evidently such an offense. That throws some standard defenses into doubt, because anything based on negating criminal intent doesn't matter. However, Michigan does specifically say that it is not criminal to have sex with a person under 16 if they are your legal spouse; this is a very common exception to statutory rape laws. So, marriage is a way to not risk jail for statutory rape in Michigan.
However, things do vary by state. In Indiana, it is specifically a defense that the defendant had a reasonable belief that the victim was over the age of consent (unless it was a forcible rape).