Robb was only 16 or so and the Stark kids all started off way too naive, honorable and unaware of the ways of the world. They were not politically savvy and thought everyone was as up front and honest as them. Robb probably did not expect Walder Frey to be too angry because he figured it was not that big of a deal - he did not understand what breaking his oath would cost him.
Oh Robb knew that Walder Frey would be mad, maybe murderously so, he just didn't have a choice. Which was his real fault, the strong get to write off their allies, not someone who relies on the Freys for a significant amount of his forces.
Walder Frey wanted his family to be married into a great house. That was his aim, he achieved it and then it was thrown in his face by the grandson of the man who repeatedly turned down his offers of marriages and mocked him. Robb was a fool, and it wasn't the honourable thing to marry Jeyne. He'd have only needed to ask his mother or father (whether his fathers answer would be truthful is doubtful) what to do in that situation. Eddard claims to have had an affair during the rebellion, possibly with a noble daughter yet he honoured his marriage contract because it's more important.
Frey maybe should have been happy with Edmure, but Rosilin (Robb probably picks her if she's attractive) would have been a Queen, being a Lady of a great house isn't the same as Queen. If he wanted to make it up he'd have needed Edmure, Brynden, Catelyn, Bran, Arya (still) and Rickon to be thrown into the deal. Edmure was never a suitable replacement for Robb.
Bran is a cripple, Cat is approaching the end of her child bearing years, Brynden... is Brynden, and Arya... maybe a good match.
Let's not forget also, Frey lost his first son, the one that he had groomed for what? Sixty years? Only to be told that despite his army making up a large part of the kingdoms force, he was being spurned for some woman, presumably because she had better blood. And then he's offered Emdure. It's gotta sting. Combine that with what happened the last time he made a match with a Great House (10 year old Tywin embarrassing him in front of everyone, I'm not sure this isn't why he rebelled in the first place, he can hold grudges) he had had enough.
And no one gives Jeyne any heat for this,she married a man that was a sworn enemy to her father and his lord, it's a miracle they all escaped with their lives really.