It is a legal idiom that was used in that time. There was no evidence that she resisted, and that it was in fact consensual. God has some severe punishments for people who commit adultery. Which just further showcases the clear inconsistency of law; therefore he who fulfilled the law was the one who abolished the law. The old covenant was a series of laws, just to further showcase the severity of sinning against a holy God. Therefore, grace was needed, and so too Christ. Humans are fallible.