(a) A defendant is guilty of a Class B1 felony if the defendant engages in a sexual act with another person who is 15 years of age or younger and the defendant is at least 12 years old and at least six years older than the person, except when the defendant is lawfully married to the person.
(b) Unless the conduct is covered under some other provision of law providing greater punishment, a defendant is guilty of a Class C felony if the defendant engages in a sexual act with another person who is 15 years of age or younger and the defendant is at least 12 years old and more than four but less than six years older than the person, except when the defendant is lawfully married to the person.