What is the criterion for necessity that allows one to adopt the easiest scholarly opinions, and does it include intense sexual desire and the distress caused by it, such as the Shafi'i permissibility of sexual fantasy?

It is permissible for a follower who does not habitually seek out concessions to adopt a concession issued by a reputable scholar when there is a need, as mentioned by Al-Subki. The author of "Islamic Jurisprudence and Its Proofs" summarized the rules for adopting the easier opinion in two matters: that the issue must be one of juristic independent reasoning (ijtihad) for which there is no preponderant evidence, and that there must be a necessity, need, benefit, or excuse.

As for masturbation, some scholars permitted it when there is a fear of illness or a concern about falling into grave sin (zina), while the majority prohibited it absolutely. Ibn Taymiyyah clarified that what was narrated from Ibn Abbas and Ahmad regarding its permissibility was for one who feared severe grave sin (zina or sodomy) to break the intensity of lust, but performing it for pleasure, remembrance, or habit is forbidden.