Sex offender is not a natural category. Registration statutes specify relevant crimes. People who urinated in public were compelled to register because statutes specified genital exposure. Buyers and sellers of consensual sex between adults were compelled to register because statutes specified consensual sex between adults.

The difficulty of proving rape is why rape conviction rates are low. Imprisonment for rape is shorter than imprisonment for murder because most people consider murder worse than rape.

People who urinated in public were compelled to register because statutes specified genital exposure in order to cover flashers but were overly broad and non-discretionary, causing an absurd result. Statutes covered flashers and prostitution because of scope creep; once you create a machine people want to use it for things. It's fairly obvious that there would be no registry to begin with if there were no rapists or pedophiles.

> The difficulty of proving rape is why rape conviction rates are low.

Even despite the low conviction rate, rape convictions are reversed at a higher rate than other crimes, because it's so difficult in those cases to know the truth.

> Imprisonment for rape is shorter than imprisonment for murder because most people consider murder worse than rape.

Many countries historically imposed the death penalty for rape and a few still do. Typically the ones less concerned about proof beyond a reasonable doubt in general.

People viscerally hate rapists and, even more, pedophiles. Murder is nominally worse but not by much. The penalty for rape and second degree murder are typically about the same. In multiple US states the penalty for rape is higher.