There are many reasons why it's barbaric. It's outdated. It's harmful to society. It elevates one bloodline as being inherently superior to others. It is a discriminatory institution. It's an institution that has a horrific history and can be readily dissolved. It's corrupt. This is not an exhaustive list.
Which father are you referring to? I have 3. 2 of them are British.
The two British ones in your parents polygamist relationship
Something being outdated doesn't make it barbaric
Something being harmful to society doesn't make it barbaric
Something that elevates one bloodline as inherently superior to others doesntmake it barbaric
Something being easily dissolvable doesn't make it barbaric
Something being discriminatory doesn't make it barbaric
Something with a horrific history doesn't make it currently barbaric
The monarchy isn't harmful to society
The monarchy isn't discriminatory