GWTW is not, in the strict sense, a romance--although one can argue that the main pairing is between Scarlett and Tara. A book that contains a romance--even a central romance--is not necessarily a genre romance novel.

Unfaithfulness may be a standard romance trope, but it usually involves someone who is discarded and frees up the H/h to find someone new.

For people who read genre romance, fidelity and HEA are usually necessary requirements, just as mystery readers expect a resolution of the mystery.