Well, technically he could just get his comfort from Jeyne and leave her and her family there (I'm speaking of book version now), going back to his army. The Freys wouldn't need to know anything, he can still marry the Frey girl of his choice and everyone is happy.
However, because of his honor, he now was bound to marry Jeyne, which he did, which pissed off the Freys.
The show version doesn't show that, it's just kinda like "oh there's that amputator-girl from Volantis named Talisa, she's also cute, let's marry each other".
(I think it's alright to compare the book and the show as all these events have already happened in the show too)
But that also is not honorable of him to do, just like Ned was always hammered for having a bastard. But yes, that would have been a better way to handle the situation in reality so he could keep his oath and also keep his lover.
I agree that the most honorable thing Robb could have done was to live up to his commitment to the Frey's, it came first. I'm certain that Ned would have told him the same thing.
I think one of Robb's motivations for marrying Jeyne/Talisa was the flack that Ned got for John Snow--which is pretty ironic because it's
nearly certain that Ned never fathered John in the first place. His unfailing honor wouldn't let him betray the promise to Lyanna that he would keep her secret. Nevermind that it made Ned look hypocritical/dis-honorable on the surface. He took that little secret with him to the grave thanks to Joffrey.
Ned's troubles somewhat mirrored Ser Jaime's too. Once you know everything that Jaime witnessed he was pretty much honorbound to kill Aerys to save the King's Landing regardless of what it did to his personal reputation.
That's one of the best thing about the Song of Ice and Fire series: All the gray areas make for more colorful characters. Fun stuff.