Whatever Thomas Jefferson may or may not have been doing with Sally Hemmings who was his late wife's half sister but also half black having had a slave mother, marriage was not encouraged between the two because the racial difference would have been a scandal in Virginia which had Brit roots.
Indeed, as to Sam Houston, he left public life rather than accept secession after he tried unsuccessfully for the Democrat nomination for POTUS in 1860.
You have an excessively rose-colored view of race relations in Latin America. Latin American countries have had less of a history of de jure racial segregation but have a greater degree of de facto segregation and hierarchy to this day. Almost all of the people in Rio de Janeiro slums are black or brown, all most all those in the high rise penthouses are European (there’s also a geographic divide with the more European affluent south of Brazil vs. the more Indian/Black and impoverished north). Similarly, most of the business elite and professionals of Mexico look like Spaniards, the peasants and unskilled workers are Indios or Mestizos. They did have the Indio-looking Benito Juarez, since then most of their El Presidentes have looked more like Vincente Fox.