It's made clear in the film that Rhett is the disgraced member of a wealthy family who makes a fortune during the war as a blockade runner. He has a lot of great lines. If he didn't, Clark Gable would have passed on the role. Here's one: "You should be kissed and often, and by someone who knows how." Then there's: "With enough courage, you can do without a reputation." and "All we've got is cotton and slaves and arrogance."
There are great exchanges with Scarlett, too, like this one. Scarlett: "Sir, you are no gentleman." Rhett: " And you, miss, are no lady. Don't think that I hold that against you. Ladies have never held any charm for me."
One other point, Scarlett is not only willing to steal a man from a friend, she steals Frank Kennedy from her sister.
Gone with the Wind will always be a great film because it was perfectly cast, the sets, costumes, script are extraordinary, and the struggles Selznick overcame to get it made are legendary. The blind eye it casts toward the evils of slavery is unfortunate, but it informs present-day audiences about how people in the past thought differently. We see Scarlett abuse Prissy, but the audience doesn't mind much since Prissy is so useless in a crisis, Scarlett's frustration is understandable if not excusable. Mammy is a great character. It may be the only example on film showing a slave who is smarter, more compassionate, decent, and dignified than her mistress. Her scenes with Rhett are wonderful.
I agree that the film should not be altered to reflect modern sensibilities. It is what it is. Despite its shortcomings, it's a towering achievement in filmmaking.