Personally, I always thought that Rhett and Ashley were carrying on behind the backs of Scarlet and Melanie. 
I believe there was a tragic scene cut from both the book and movie that would have gone like this…
“‘Frankly my dear, I don’t give a damn’” Rhett quoted.
“You said that to her?” Ashley replied, stunned at the use of such vulgar language, especially in the presence of a lady. He paused for a moment, the bottle of scotch in his hand. Then poured another dose for himself and his “guest.” It was good to have Rhett here, especially after Melanie’s passing. Men like him were needed to console each other, especially in their time of need. He would miss Melanie deeply, but he was afraid he would miss Rhett even more, when he left for Charleston.
Rhett stood up and quickly downed the shot of scotch, savoring the flavor as his eyes fell upon the unclothed figure of Ashley Wilkes. Ashley’s skin, pale in the yellow candle light of his bedroom was in stark contrast to his own darker, animal-like body. What a strange contrast, he thought again, for the thousandth time. Ashley, so much like the old south that had faded away, pale before the aggressive power of the future could never have Scarlett now, nor would she ever want him again. And yet, she had had her chance to have him, Rhett Butler, the lionising captain who could be a real man for her, the real future she needed and yet she had thrown him away, too.
“I’ll be back, Mr. Wilkes!” Rhett said, a sparkle in his eye as he said it. “With your dear Melanie gone,” here he lowered his eyes a moment, “and my separation from Scarlett, there’ll be even more opportunity to see each other. Did Melanie ever have any idea how close we were?”
“She suspected,” Ashley said after a moment’s hesitation. “But Scarlett’s infatuation with me was more than enough to obscure any hint of impropriety between us. Our secret is safe, Rhett!”
Captain Butler quickly dressed and left the Wilke’s residence unnoticed, heading for Belle Watling’s house. He would be back to console Ashley, after all, tomorrow is another day!