Hold up

Maybe I'm not understanding your confusion. But I'll try to answer it. Sorry if I didn't get your point.
He said Chuu was his fiancé because he liked him and wanted to take responsibility for "doing him" when he was fourteen (they never did it, but he was in denial).
As he wanted to may him but he was too young, he thought of him as his fiancé. Also he wanted to wait for Chuu graduating from college, if I'm not mistaken.
And the adoption is the way he was going to marry him, because same sex marriage isn't aloud in Japan, so they use it as a legal method to create a connection between the couple and acquired some rights as a family.

*marry him.
Also he thought Chuu was his fiancé (I say thought because Chuu always rejected him) because he asked permission from Chuu's father and the latter made an agreement with him (become rich) but Chuu said his father was joking.

Yeah I got that part xD And Homosexual Marriage would be illegal during that time (or maybe still ). It 's just that Shiratori was throwing out that Chun was his fiancé, like the two of them were official even Chun had no say in it or didn 't even know . But the whole I completely forgot about the him going to Chun 's father about it , that would make sense then xD
And also the adoption thing, idk I just thought it was kinda... Not so weird but like I never would of thought of that as another way to have someone you love under your name if Gay marriages were illegal. Ah and since Chun IS technically an Adult where the present manga takes place (they ARE in college right ? and I 'm not sure if 18 is considered an adult in Japan but ), would you still be able to "adopt " someone ?

Thanks for explaning !

I think they become legal adults at 20.
I don't think it matters if he is an adult or a kid, adoption is allowed.

You are welcome :)

In japanese society to ensure inheritance and to insure you have a "son" to perform your burial rights you can adopt anyone of any age into the family registry.
Having a male heir is important religiously as I understand it. If a male heir doesnt perform your burial rights you can not pass on? Thats what I have learned from my aunt who lived in Japan for a few years but that might not be the only reason.

In most Asian countries there's no official same sex marriage (yet). So in order for your same sex spouse to have legal family rights (health insurance benefit, inheritance and generally making some bureaucracy related stuff easier) you enters him/her into your family registry or vice versa. I don't know how it works in Japan but in my country it's not that hard to add someone into your family registry.

And in my country you can have your own family registry if you are officially married (however young your age is) or if you are of legal age+have postal address+have reference letter from head of community where you reside.