Especially with the producers saying that it was a family thing with Nick/Adalind/Diana/Kelly. That line if Diana was genuine comes back to how she was probably raised and taught to think. I wish they had chosen another line. 2
See my previous answer for the reasons why it may not be Adalind and Nick she’s referring to when Diana says “mom and dad.”
That being said, the rest of your point stands. I find the line creates a lot of disconcerting possibilities around how Kelly and Diana grew up, and what they learned from their parents and the other adults in their lives. Then again…even the most well-meaning parents can sometimes instill prejudices in their kids.
I mean look at how quick Monroe was throughout the series to stereotype different Wesen, even as a Wesen who had consciously and deliberately made a choice to do something different with his life than the “old world” way. Even after being a victim of the ugliest forms of prejudice in his world and nearly losing his life, he was still quick to judge and pigeonhole other Wesen in certain situations.
So I see how it could happen even if Nick didn’t intend it to…but I still really, really wish they had chosen a different line.