extramarital_sex_exposure_consequences_with_target_effect

extramarital_sex_exposure_consequences_with_target_effect = { #Note: only adds stuff for one of the sex partners, it must be run twice! It is intentionally kept this way so you can apply it in two different places in an event chain etc.
	save_temporary_scope_as = sex_character

	#Applies traits, piety level loss, unfaithfulness opinions
	$TARGET$ = { save_scope_as = sex_partner }
	if = { # Check to make absolutely sure it's extramarital
		limit = {
			NOT = { scope:sex_character = { is_consort_of = scope:sex_partner } }
		}
		if = { # Check to make sure your culture isn't A-okay with this
			limit = {
				NOT = {
					culture = { has_cultural_parameter = adultery_always_accepted }
				}
			}
			extramarital_sex_exposure_consequences_effect = { SPOUSE_EVENT = $SPOUSE_EVENT$ }

			#Slept with spouse opinions & potential rivalry
			scope:sex_partner = {
				every_spouse = {
					limit = { this != scope:sex_character } #Extra safety
					save_scope_as = sex_partner_spouse

					hidden_effect = {
						#Remove duplicate opinion from discovery
						if = {
							limit = {
								has_opinion_modifier = {
									target = scope:sex_character
									modifier = slept_with_spouse_discovered_opinion
								}
							}
							remove_opinion = {
								target = scope:sex_character
								modifier = slept_with_spouse_discovered_opinion
							}
						}
					}

					if = {
						limit = { accepts_adultery_without_penalty_trigger = no }
						if = { #(only visible for directly relevant exposure)
							limit = {
								always = yes
							}
							add_opinion = {
								target = scope:sex_character
								modifier = slept_with_spouse_exposed_opinion
							}
						}
						else = {
							hidden_effect = {
								add_opinion = {
									target = scope:sex_character
									modifier = slept_with_spouse_exposed_opinion
								}
							}
						}
						hidden_effect = {
							#Rivalry
							if = {
								limit = {
									is_ai = yes
									scope:sex_character = { is_ai = yes }
									ai_vengefulness >= medium_positive_ai_value
									can_set_relation_nemesis_trigger = { CHARACTER = scope:sex_character }
								}
								set_relation_nemesis = {
									reason = nemesis_extramarital_sex_exposure
									copy_reason = rival
									target = scope:sex_character
									involved_character = scope:sex_partner
								}
							}
							else = {
								scope:sex_partner = { save_scope_as = relationship_reason_involved_character }
								progress_towards_rival_effect = { CHARACTER = scope:sex_character OPINION = 0 REASON = rival_slept_with_spouse }
								clear_saved_scope = relationship_reason_involved_character
							}
							# HOUSE RELATIONS
							if = {
								limit = {
									exists = house
									exists = scope:sex_character.house
								}
								house = {
									change_house_relation_effect = {
										HOUSE = scope:sex_character.house
										VALUE = house_relation_damage_medium_value
										REASON = cuckoldry
										CHAR = scope:sex_character
										TARGET_CHAR = scope:sex_partner_spouse
										TITLE = scope:dummy_gender
									}
								}
							}
						}
					}
				}
			}
		}
	}
}

As written in game/common/scripted_effects/00_adultery_effects.txt.