I understand equal protection has been argued before with lititle success, however most arguments revolve around whether punishment (registry) is unfairly applied to one group. And, that argument typically gets diverted to the “it’s not punishement, because it’s a civil regulation” BS.
In this case, however, the state is essentially granting very favorable 1st ammendment (right to vote) rights to all but SO’s. And, that fact, cannot be reasonably defended by the commerce clause. It’s discriminatory, and a violation. It’s not based on a felony; and, of course, it’s not based on reason.