Why isn't the punishment for a rapist sufficient, and what is the evidence from the authentic Sunnah regarding the Prophet's (PBUH) ruling on the rapist?

If the rape involves coercing a male into sodomy, the punishment for the coercer is execution. There is no distinction whether the sodomized individual was willing or unwilling; the perpetrator's punishment is stoning unconditionally, regardless of whether the sodomized individual was an adult or a minor, willing or unwilling.

If it involves coercing a female into adultery (zina), the punishment for a married rapist is stoning to death. For an unmarried rapist, it is one hundred lashes and exile for one year, in addition to the customary dowry (mahr al-mithl) for the woman. As for coercion to acts of sexual pleasure short of intercourse, there is no prescribed Shar'i (Islamic legal) punishment (hadd), but rather a discretionary punishment (ta'zir) determined by the judge to be a deterrent.

The Prophet, peace be upon him, ruled with stoning for one who coerced a woman into adultery, and Allah granted her forgiveness. Abd al-Malik ibn Marwan ruled that the coerced woman should receive her dowry. If the crime is committed with brazenness and defiance, the punishment for hirabah (brigandage/unlawful warfare) mentioned in Surah Al-Ma'idah is applied. The punishment for hirabah encompasses life, property, and honor.