Idk about the next story. But one partner adopting the other in same-sex relationships in countries that don't have legalized same-sex marriage is the only way to have any rights akin to a married couple. It's the only way to be each others family's in the eye of the law. It's nothing about actual incest but instead being able to make decisions for your partner in case of an medical emergencies or having rights over their assets in case of a fatally. Basically he's proposing to Yuta.
Well they could be married in a country that recognizes same-sex marriage but in Korea and Japan internationally recognized same-sex marriage are not recognized. The old man's family might treat him like they were legally married but he probably was adopted into the family too if he had any right over the guy's patrimony.

Guys is KinGin's next story going to be incestous? Because it’s going to be about the twins...I hope KinGin won't do something disgusting like that