game_rule_sexuality_distribution_reroll_effect

game_rule_sexuality_distribution_reroll_effect = {
	if = {
		limit = { 
			age >= 10
			NOT = { is_from_ruler_designer = yes }
		}
		random_list = {
			0 = {
				modifier = { add = heterosexuality_chance }
				set_sexuality = heterosexual
			}
			0 = {
				modifier = { add = bisexuality_chance }
				set_sexuality = bisexual
			}
			0 = {
				modifier = { add = homosexuality_chance }
				set_sexuality = homosexual
			}
			0 = {
				modifier = { add = asexuality_chance }
				set_sexuality = asexual
			}
		}
	}
}

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