Because in some country same sex mariage is not legaly reconised and it's the only way to be on the family registrer ╮(￣▽￣)╭
Adult adoption is just a loophole-y way to make sure the partner is legally recognised as an heir in case something happens. Things like medical decisions or property transfers, being a dependent on the insurance or nominee for investments need the person to be family - either by blood, marriage or adoption.
So, even if they don't recognise a same-sex partner, they can't not recognise an "adopted" family member.
Property inheritance, not transfer
BUT WHY DID HE HAVE TO ADOPT HIM AS THE SOLUTION TO BEING TOGETHER??