This is true
Carl and Lenny clearly love each other but cannot express it in Springfield
You will notice when HOMER moved in with the two gay guys it was the WHITE guy not the LATINO guy that kissed Homer.
Smithers had to pull a Michael Jackson and get his skin bleached.
As hard as it is for people to believe until very recently America was not accepting of mixed races. The 70s Show is laughable with their inter-racial dating. It would NOT have happened in that time frame.
Remember as late at the 1989 Fox had a show about a black man and a white woman (I forget the name it had Nancy Walker in it. Maybe it was called "True Colors?") and that was a big deal.
That said Springfield is a small town and you really don't see much more than Token races anyway, so perhaps it isn't a racial thing as much as a lack of opportunities.
I can think of Three African-Americans, Cosby clone --Dr Hibbert, Carl and the boxer guy Tatum, with his sterotypical "motherS of my children."
Asians -- Apu and his family and Cookie "the west side is mine."
Oh I just thought of one. The Fugu episode. Edna is getting hot and heavy with the Asian guy in his car.
BTW Darren Hayes was married to a white person, just like Homer