when you turn 10 you get a mark somewhere on your body. tanjiro did not care about the gender of his soulmate. considering he's pansexual. when he got his mark he was so happy and was jumping around with exciment his family thought that his head would fall off. zentiu was a ally of the LGBTQ+, as he wanted his soulmate to be a girl but supported the community. when he got his mark he thought is was weird but did not mind it and was happy. thats basicly whats its about but i suck at describing soooooo lol