The interpretations of why Emil said it which people pulled out as soon as other readers started complaining were in very bad taste, and I think your view and decision is fair.

The in-universe reasoning is absolutely meaningless in a throwaway joke. It's only the reasoning of why Minna wrote it that matters, and that reasoning is...distasteful at best, particularly combined with a refusal to change it because it's 'good-natured ribbing' (Between who? Write your character's interactions as you please, but the only real conversation in writing is between you and your audience. Minna's dismissal screams to me of a certain assumption that the people who matter will not be offended.)

I'll probably continue reading, but with considerably less faith in and fondness for the author. And no longer going to buy that book.