The Missouri statute you are referring to is 573.110 (Nonconsensual dissemination of private sexual images). A conviction under that statute is a Class D Felony as you stated, but is not a sex offense that requires registration.
That said, the authors of the statute fully botched it and (in my opinion unintentionally) made it extremely difficult to prosecute cases where the image shared wasn't shared specifically for revenge. In order to gain a conviction, the state must prove that the person shared the image with the intent to harass, threaten or coerce the person depicted in the image. Unfortunately, the intent element would not necessarily be met in the scenario where a person confesses "I sent this graphic, private sex tape to my buddy without the girl's consent because I thought my buddy would like it and I didn't think she would ever find out." They marketed this bill as a response to revenge porn where people were posting stuff online to get even with exes (or scammers who used the threat of leaking nudes to get money out of people). But in doing so, they didn't craft it to include secretly filming another in the act and then passing it around to your buddies for clout. I know several prosecutors who are extremely frustrated with this statute. It is ridiculous that "she didn't even know I was filming and I never thought she'd find out I sent it to my friend" is an actual defense to this statute.
If he had done exactly what was alleged/admitted in Missouri, I don't believe that he would have violated a criminal law. He'd be subject to a civil suit, but I do not believe that he would be convicted under the statute you are talking about. I think the state may have been able to make a harassment case for his communications with her after the fact, but those details are murkier and it would have been very tough. Realistically, if Missouri tried to go at him as hard at they could, he definitely wouldn't have gotten prison, he might have gotten probation, and he very likely would have gotten an opportunity to keep a conviction off his record.
None of that changes my opinion of his conduct and I'm not trying to change your opinion on his conduct.
But in a large number of areas (including the state the Blues play in) what he did is not a sex offense, does not require registration, would almost never result in prison time for a 1st offender, and in many areas is not even a criminal offense.