I think you are underestimating what attitudes there were in “the most liberal families” in 1960. I was actually in one at the time! The Dunhams went to the “Little Red Church,” a Unitarian Universalist church (name says it all), and the Mercer Island HS was an island of Marxist third-world liberation and diversity-loving enthusiasts, it would seem.
No doubt HI and WA are liberal states, and the Dunhams were quite ultra liberal....but even a liberal white family (esp one comfortably middle class like the Dunhams) back in that time would not have publicly made available their 18 yr old daughter with her half white/black child....with the father nowhere around.
The Dunhams had Obama living with them as their grandson for many years. They didn’t act embarrassed about it.