Symbolism, Imagery, Allegory
Dark and Dangerous
When Rhett first appears in the novel he's described as "dark of face, swarthy as a pirate" (6.16). Rhett is a white man, but he looks like he's black—and in a novel as racist as this one, that's an important detail.
Rhett's darkness directly links the novel's fascination with race to its fascination with sex. In the novel, most of the "good" black people—those who remain loyal and who (you're supposed to believe) enjoy being slaves—are asexual. Mammy and Uncle Peter appear to have no sexual interests at all; they happily spend their lives serving white folks, never once even thinking about the possibility of falling in love themselves, or raising their own families.
Even the flighty Prissy seems uninterested in flirtation. The sole exceptions to the whole good-black-people-don't-have-sex equation are Prissy's parents, Pork and Dilcey. Prissy's living proof they have had sex at least once, but we never see much of their relationship, and we certainly never see the two of them alone together.
But while "good" black people are desexualized, "bad" black people—you know, those who want freedom—are presented as uncontrollably hyper-sexualized. Eustis, the Fontaine's freed slave, just speaks to Sally Fontaine, and this is treated as a rape, requiring the murder not just of Eustis but of the white man who "had the gall" to say that black people "'had a right to—to—white women'" (37.20), at least according to the racist murderer Tony Fontaine. Interestingly, Eustis has been freed at the time of this incident—and when he dares to act as such, he gets killed.
Later in the novel, Scarlett undergoes a sexualized attack from a black man who fondles her breasts (in an effort to get at money), prompting Scarlett to feel "terror and revulsion such as she had never known" (44.108). When Scarlett works her sexuality to her advantage, though—often to get money for herself—instead of inspiring immense fear and disgust, she usually just lands herself a husband. Her sexuality, then, is a socially acceptable tool to some degree; the black man fondling her breasts, however, is a person wildly outside the confines of their station.
But back to Rhett and that initial description. By referencing pirates and a dark skin tone, Rhett is doubly introduced as an outlaw, a man who takes what he wants. And he does nothing to work against these stereotypes, even going so far as to (arguably) rape his wife.
Evil Sex
Black sexuality is either completely denied or violently rejected as animalistic and repulsive. But… Rhett suggests that black sexuality is not just repulsive.
Rhett is dark, like the former slaves, and he's animalistic and dangerous, as they are supposed to be. But his dangerous, brutal sexuality is presented as exciting and stimulating. When Rhett, in a jealous rage, attacks Scarlett late in the novel, Scarlett finds she enjoys the violence, which she describes explicitly in terms of blackness:
She was darkness and he was darkness and there had never been anything before this time, only darkness and his lips upon her. (54.67)
Interracial sex, then, seems like it's horrifying in part because it's so desirable and forbidden and exciting. On the one hand, the novel suggests that black men who are not entirely asexual need to be systematically exterminated, while on the other, it thinks its romantic lead is exciting because he's "dark."
It's not black people in the novel who lust after white people, but white people who indulge in sensationalistic sexual fantasies about black people. And to make up for those fantasies, the novel has to send white guys like Tony Fontaine to kill black people. Which doesn't really seem fair—but it's just one more reminder that a society built on slavery is not a very nice place.