According to an article entitled "A Cultural Context for Othello" written by a theater historian for the Shakespeare Theater Company at the Harman Center for the Arts, in Shakespeare's day, Moors were generally regarded as being morally corrupt, jealous and sexually promiscuous and were spoken of as villains and devils. On the other hand, Venetians were regarded as counterparts of English society.
The term "Moor" came from the country named Mauritania, but it generally referred to all North Africans and West Africans and, for some people, all Muslims and non-whites. In early 17th century England, Moors were ostracized for their unusual clothing, customs and habits. Whites cited Christian scripture to justify their concepts of white as pure and black as evil. Though blacks were not thought of as slaves, as the slave trade was not well-established until the late 17th century, they were shunned as bestial and dangerous. At first, Queen Elizabeth gave diplomatic recognition to the Moors as her allies in vanquishing Spain, but later she deported them, fearing they would overpopulate the country.
As "A Cultural Context for Othello" points out, Venice is used as a substitute for London in the play, and the attitudes of the Venetians mirror those of the British. The Venetians, therefore, appreciate Othello's leadership and courage, but at the same time are horrified at the notion of him marrying into white society.