Involuntary Arousal (Taḥarruk al-Sākin) 2

Answer—and Allah is the Guide to success: As it appears to me, if the arousal occurs without a deliberate cause, nothing is due upon the muḥrim—and likewise if it occurs with no cause at all—by the proof of His saying, “Allah does not charge a soul except [with that within] its capacity.” [al-Baqarah:286] and, “And there is no blame upon you for that in which you have erred…” [al-Aḥzāb:5]. Involuntary arousal often occurs without a cause—merely due to the body’s natural state—and it is not within a person’s capacity to prevent that. Arousal may also arise from inner talk, and one cannot rid himself of much inner talk. If the arousal arises from such inner talk which the legally responsible person cannot ward off, then nothing is due.
What does require a sheep is arousal that results from a deliberately chosen cause by the muḥrim—such as looking with desire, kissing with desire, embracing with desire, or deliberately provoking lustful thoughts. This, it seems, is what the scholars of the madhhab intended. The proof for the prohibition of that in ḥajj is His saying, “So whoever has made ḥajj obligatory upon himself therein, there is to be no sexual relations (rafath), nor disobedience, nor disputing during ḥajj.” [al-Baqarah:197]. Rafath is the sexual speech that preludes intercourse between spouses; to it are analogically attached the other preludes to intercourse—kissing, fondling, embracing, and lustful staring—because they all invite to intercourse and stir desire.
The evidence for considering this shared cause is inductive: we observe that the Lawgiver forbids, for the muḥrim, every cause that invites to intercourse—so He forbade adornment and perfume, the contract of marriage, rafath, and so on as is detailed in this chapter.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1