What is it with japanese manga and like brother's love/cousin's love/uncle's love! Please ...

it's like a real subculture in japan !! someone educate me if i get some things wrong but gay people in japan often adopt the spouse who's younger in a couple so that they can live in the same household and share benefits just like how couples would do..? they get to share some benefits together (u can search for it) and live in the same roof and much more. since same-sex marriage still isn't legal in japan i think this is one of their ways to be legally be together! although it still gives me uncomfy vibes but i think that's where they got their obsession with step-relative trope (?) there could be other reasons tho..