Many of these beliefs were taught in churches and in society in general.
Races were separated in the '40s and '50s. There were separate public restrooms, business establishments and restaurants created for each race. In the '40s and early '50s, it was simply understood that a person of one race did not enter the establishment of another race. In the '60s a mixed-race child was ridiculed and looked down upon. Society has advanced since then, so this type of behavior is not often seen, making it more acceptable to date someone of a different race.
In the 1960s public schools were integrated, helping different races to become more tolerant of one another, even if they didn’t fully agree, and to become friends in many cases.
The newer generation has not experienced the turmoil of a separation of races. To them, all persons have been equal since they started attending public schools. They were not taught racism. From this, people of different races were better able to see that no differences exist between the races, making it more acceptable to date one another.
Ancestral heritage can play a part in who a person chooses to date and marry.
A person who has parents, grandparents or other relatives comprised of different races generally feels more comfortable dating people of various races.
During the years of slavery in America, it was common for male slave owners to have sexual relationships with their female slaves, which resulted in a lot of mixed-race children.
In Virginia in particular, there is local lore that Thomas Jefferson had slaves of Indian as well as Negro descent. It is said that he housed the Indian women with the Negro men and the Negro women with the Indian men in the slave quarters. It also is thought he may have fathered children with one of his slaves. However, the truth of this matter isn't definitive.