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It's not just bl, people irl do it as well. 'Same-sex adult adoption involves adult adoptionāthe adoption of one adult by anotherāof a partner in order to benefit in some way, such as to create family relationships, to ensure inheritance rights and to keep collateral relatives from contesting the estate plan of the adopted adult.'
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I think it's not the same as in western countries. They are in the same family registry and it doesn't necessarily make them father and son (by law). But definitely take what I said with a grain of salt since all my knowledge is just throu reading and not because I know it first hand. What I do know for sure is that it really happens in reality since thats the only possibility they are legally tied together.
Why would he adopt him