That letter could have been written about my family of origin. They are a hateful and bigoted bunch, but excuse it all under the umbrella of their religion. My grandmother calls people the N-word. My mother and sister are nasty and hateful about gays, other racial groups, single parents, people with mental disabilities, and much more. They are not as overt about it as the situation in the letter, but if you're around them long enough you'll get an earful.
I think the "apple doesn't fall far from the tree" argument is hurtful. I make every effort to be kind, compassionate, and respectful of every fellow man.
However, I think it's seriously problematic that the boyfriend was "nonapologetic". It shows that he is tolerant towards bigotry and that would be a serious problem for me. I know that many people are defensive about their families (my DH is one of them) but if he can't see the issue with outright hate speech, I would consider that a serious flaw. Were I the LW, I would have expected (a) a warning beforehand, and (b) an "I'm sorry you had to listen to that. I agree that they're incredibly rude, and I don't blame you for being upset."