In GUESS WHO'S COMING TO DINNER, the newspaper owner husband has deep reservations at first about the daughter's wanting to wed an African American, accomplished though the man may be. Poitier's doctor is a widely respected and published one. When the daughter and her fiancé have dinner and drinks with another couple, the couple expresses its appreciation for the tough social causes her father's newspaper took on. I'd forgotten about that short but key scene. When Matt (Tracy) has a knee-jerk reaction to his daughter's marriage plans, it causes friction. When Christina (Hepburn) reminds him that they taught their daughter that black, brown and other people of color were just as good as white people, they didn't add -- "but just don't marry one." She supports their daughter, Joanna.
Dr. John Prentice (Poitier) has parents in Los Angeles who fly up to San Francisco to meet the fiancée, her parents, and to have dinner. They have no idea she's white until they land at the airport and meet her. John's dad, like Joanna's dad, has reservations. The two mothers are in sympathy with each other.
He hates the dark elements still at play in our Land of the Free. He's afraid for Joanna and for John. But, as the family friend and Catholic priest tells Matt, "They'll change this stinking world."
As for Dr. John Prentice, we tend to forget that the marriage will be his second one. He was married for about five years. His wife and son were killed in an accident. For nearly ten years, he was a widower who racked up an impressive list of credentials -- probably becoming an over-achiever in his profession to deal with his grief and numb the pain of being a childless widower. We can tell from the way the parents react that the first wife was Black.
Yes, there is sentimentality and Hollywood legend at play in GUESS WHO'S COMING TO DINNER. But, considering the backdrop of American history the year it came out and the night it won Oscars, it was pretty bold for its time.