The question answered!

The question of mine that lead to the question of my brother's that lead to this post involved a quote I'd seen in some news article. I cannot find the exact quote now, but it was basically: "Marriage didn't evolve from the belief that women are inferior to men." The context of this was that the speaker was trying to dismiss any claims of equivalence between the state of gay marriage now and interracial marriage some decades ago--the clear implication being that anti-miscegenation laws grew up around the belief that blacks were inferior and whites superior.

And as I detailed in my post, that was surely a large part of it. However, there's more to it than that. The first black servants landed in Virginia in 1619, and the first known punishment of interracial sex occurred a scant 12 years later. The first anti-miscegenation laws were passed before the end of the century--an excellent illustration of this is the interactive map at the official website of Loving Day.

However, beliefs in black inferiority weren't cemented nearly so early. Says George Fredrickson in The Black Image in the White Mind:

In the years immediately before and after 1800, white Americans often revealed by their words and actions that they viewed Negroes as a permanently alien and unassimilable element of the population. Yet articulate whites of that period were characteristically unable, and perhaps even unwilling, to defend their anti-Negro predispositions by presenting anything that resembled a "scientific" or philosophical case for the innate moral and intellectual inferiority of the black race. In the 1780s, Thomas Jefferson, alone among the spokesmen for the American Enlightenment, had moved in this direction by arguing that blacks were probably inferior to whites in certain basic qualities, but he conceded that all the facts were not available and that final judgment on the question ought to be suspended.

"The opinion that they [the blacks] are inferior in the faculties of reason and imagination, must be hazarded with great diffidence," he wrote. No hypothesis could be verified until more scientific investigation of racial differences had been carried out: "I advance it therefore as a suspicion only, that the blacks, whether originally a distinct race, or made distinct by time and circumstances, are inferior to whites both in body and mind." In the words of Winthrop Jordan: "Until well into the nineteenth century Jefferson's judgment on that matter, with all its confused tentativeness, stood as the strongest suggestion of inferiority expressed by any native American."

In the 1780s the following states already had anti-miscegenation laws: Louisiana, Georgia, North Carolina, South Carolina, Virginia, Tennessee, Maryland, and Massachusetts. Further, Pennsylvania had one but repealed it in 1780, and New Netherland also had one before it became New York.