I think there is a difference between sympathies and opinions/predictions.

Unless there was pretty special circumstances, I'd probably always root for a Danish fighter against a non-Danish fighter. Even if I didnt really like the fighter in question that much. [This might be due to coming from a small country, where there is only so many boxers to choose from - if you're American it's easier to 'afford' not caring about certain fighters].

This doesnt mean though that I dont try to keep an objective, unbiased opinion. [I may not always succeed in this, but at least I try].

In short: Nationalism in opinions on boxing is stupid. But favoring certain boxers because of nationalism [1] is not necessarily stupid IMO.

[1] I actually had a course on nationalism a few years ago. Here they distinguished between patriotism - liking your own country - and nationalism - disliking (people from) other countries. If this definition should be applied - I found it a bit stupid and simplified to be honest - nationalism is obviously a bad thing, while it is patriotism that I think should be allowed.