My gf and I are poly. She has a gf. It doesn’t bother me at all. You should talk to your husband; his resentment isn’t necessary. It sounds like everyone (your daughter, her bf, his gf) are all aware of the situation and consenting. What’s the issue?
Same reason why people from the United Kingdom are called ‘British’, despite Northern Ireland not being in Britain.
There just aren’t better proper adjectives for these 2 countries.
While you can say ‘North American’ to mean anyone from North America and not specifically the US, I’m not sure there’s a fitting word that refers to anyone from North or South America. Although, at that point, the group of people you are talking about is perhaps too broad to be useful in most cases.