This guy has done 3 years, maybe it should be more. But, all the court is saying, is that if he had sex with a 14 year old girl, it would have been 15 months.
The court is specifically not saying that you can't increase the sentence to 17 years if you so choose. Only that it has to apply to everybody. The Kansas legislature is quite free and able to change the law to reflect that if they so choose.