Seems from your specific example that the kiss is happening to the woman rather than her being an active participant in it. She even struggles a bit against it at the beginning (maybe she's being coy). Either way it's probably related to Hollywood's view of women's sexuality evolving from something they put up with to something they engage with, reflecting cultural and social norms of the time.

While I did not see Gone With the Wind myself (the example scene you provide) I found an interesting analysis of the gender dynamics. Here's a quote:

"What Scarlett sees in Ashley remains a mystery throughout, especially because she despises stereotypically feminine traits in everyone else, including other women. This mystery is encouraged by the film's dynamics because it offers Rhett as the perfect match for Scarlett and suggests that her inability to reconcile herself to this match underscores her failures as a woman. Further, Rhett and Ashley are not merely different: they represent opposite ends of the masculinity continuum as our society defines it (Taylor called them "the king and the wimp"8). The most vivid image of this antithesis is Rhett carrying the wounded Ashley to bed to receive treatment. It is tempting to suggest the encoding of a pseudo-Freudian message: Scarlett the girl fell in love with Ashley and Scarlett the woman supposedly loved Rhett, implying that as she reached maturity she recognized and desired him who could dominate her "properly." Perhaps this reading stretches the point (although Rhett himself tells her he's waiting for her to "grow up" and get Ashley out of her mind), but the film's equation throughout is that masculinity = ability and willingness to dominate a woman = sexual attractiveness. Correspondingly, the stronger or more independent (read masculine) the woman, the more masculine the man required to conquer her. Clear Ashley (and Charles and Frank) will not do. "

Source: http://www.imagesjournal.com/2002/features/gwtw/default-nf.htm

So the kiss in question may be the filmakers take on "masculinity" which seen through the lens of social norms of the time translates into domination. Modern films are more confortable with women owning their sexuality and therefore engaging in and participating actively and willingly in passionate kisses.