In some Asian countries - it is not possible to have same sex marriage. So the compromise that people use is to "adopt them" because in the Asian culture being family is the key to marriage.
Since you are adopted, you are now family and thus have "created" a marriage. It is a very common cultural/literature theme :)
The objective is for them to have the same surname and so that misaki will officially be part of the family. Same sex marriage is not legal in Japan so that’s the compromise they use. Technically in paper they’re brothers but they’re married in the eyes of their family
Because gay marriage isn't legal in japan yet, adoption into the family is the closest you can get. It gives Misaki all the privileges of a spouse, without the title (hospital visits, etc.) That was him proposing to Misaki <3
Why adoption paper and not marriage.... I mean if arikawa's mom adopt misaki then willn't they become brothers or maybe not...... I am confused