I don't buy that at all. A single moment of weakness (which is not something I find less plausible now that he's a few years older; people still impulsively hook up) is inherently much more plausible than this exceptionally calculated act of ruin.
I wouldn't call it an exceptionally calculated act of ruin. He just doesn't give a fuck about the Freys and thinks that they should be happy to shut up and take what he gives them. And he's right. All of Walder Frey's potential recourses do nothing but fuck the Freys over. He doesn't consider the possibility that Walder is spiteful enough to destroy his own house just to get revenge for a minor slight (and make no mistake, the Red Wedding did destroy House Frey; everyone hates them and no one trusts them, it's only a matter of time now before they fall).
Rob didn't want to marry the Frey girl. He never really intended to marry her. He agreed to it because he needed to move his army over the river, but both sides knew that he had absolutely no reason to keep his word once his army had moved through, particularly since the price was unreasonable in the first place.