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.
That isn't true at all. He intended to marry her, and it was hardly unreasonable. Frey didn't just let him cross a bridge, he gave him thousands of soldiers in alliance (he's House Tully's largest bannerman), and committed his House to Robb's cause. Even if Frey hadn't gone the route of the elaborate betrayal, he could just as easily have just joined Tywin openly and denied Robb the use of his fortress, severely impeding his army.
It's suicidal for Robb to cavalierly break oaths like that, and contrary to the Stark character. It's the reason nations don't discard treaties on a whim; it means your word has no value. In-show, he'll get pretty much what he deserves.