I agree with you. The best possible advice is to trust your partner and be open to them and tell them everything. Even if it might hurt them, honesty is the least they deserve. It also makes everyone involved feel way more comfortable around each other, and happier overall.

Nationalism always goes back to racial superiority. Hence why it sucks. Nationalism, in surface, is mostly a matter of national sovreignty and social legacy over all other matters. Be it the individual, geopolicies, economics and etc. The problem is when "social legacy" goes to the road of "culture", and subsequentially "race". Although culture and race might be part of National Identity, it doesn't excuse condemning miscigenation under the premise it overrides National Identity, because they don't. When different races mingle, the offspring carry some of both's traits, hence better to contribute to our nation. And it is specifically contradicting being a Brazilian Nationalist, since we have literally "no identity" in that regard.