What is Satan's goal in tempting humankind with the lust of the private parts (masturbation and fornication) in this worldly life, and what are the harms and consequences of exploiting it in the forbidden?

The devil's ultimate goal is to tempt the children of Adam by every possible means, starting with polytheism and ending with inflicting harm upon the righteous, including the pursuit of sexual desire and its indulgence in the unlawful. The Prophet, peace and blessings be upon him, warned against the desires of error in the bellies and private parts, and the misleading allure of trials. Succumbing to these desires is an act of disobedience to Allah the Almighty, exposing the one who commits it to His wrath and punishment in this world and the hereafter. It deprives them of Allah's success and assistance, and places them among the allies of Satan. Adultery has severe punishments and dangerous consequences for the individual and society. Masturbation is also forbidden, and persistence in it may elevate it to a grave sin.