can_take_as_concubine_character_trigger

can_take_as_concubine_character_trigger = {
	save_temporary_scope_as = can_marry_check
	can_marry_common_trigger = yes
	$CHARACTER$ = { can_marry_common_trigger = yes }
	#Opposite genders if you don't have accepted same-sex marriage game rule enabled and your faith supports it
	trigger_if = {
		limit = {
			$CHARACTER$ = { allowed_to_marry_same_sex_trigger = no }
		}
		sex_opposite_of = $CHARACTER$
	}
	#Faith hostility & consanguinity
	trigger_if = {
		limit = { NOT = { is_courtier_of = $CHARACTER$ } } #If you're someone's courtier, your liege can marry you anyway
		rite = {
			rite_allows_marriage_consanguinity_trigger = {
				CHARACTER_1 = scope:can_marry_check
				CHARACTER_2 = $CHARACTER$
			}
			#faith_hostility_level = {
			#	target = $CHARACTER$.faith
			#	value < faith_hostility_prevents_marriage_level
			#}
		}
	}
	trigger_if = {
		limit = { $CHARACTER$ = { NOT = { is_courtier_of = scope:can_marry_check } } } #If you're someone's courtier, your liege can marry you anyway
		$CHARACTER$.rite = {
			rite_allows_marriage_consanguinity_trigger = {
				CHARACTER_1 = scope:can_marry_check
				CHARACTER_2 = $CHARACTER$
			}
			#faith_hostility_level = {
			#	target = scope:can_marry_check.faith
			#	value < faith_hostility_prevents_marriage_level
			#}
		}
	}
	NOT = {
		scope:can_marry_check = { is_spouse_of = $CHARACTER$ }
	}
}

As written in game/common/scripted_triggers/00_marriage_triggers.txt.