But all the supreme court did in this case was decide that in the case of a willing 18 year old student, it's not criminal.
It was criminal (teacher student relations), it was not a case of statutory rape. The same would hold if she had been 16.
The way the law was written identified it as being with a “minor”. Age of consent being 16, the age of majority there is 16.
“18=adult” had nothing to do with the decision.