The only reason Robb DID marry Jeyne was because he'd already slept with her and he didn't want her to have a bastard like Jon. This is kind of presumptious on Robb's part, being that Robb is the one who ends up being dead, and Jon ends up being Lord Commander of the Night's Watch who is more than likely going to play a huge part in how Westeros fares through this war since it's more than likely that some serious **** with the Others is going to go down. However, you have to think about Jon's parentage, and think on Ned's character in remembering the stories they put out about Jon's parentage. Ned Stark would never have cheated on Catelyn, no matter what. He was far too honourable to do something like that, no matter his own feelings. On the other hand, the one thing that Ned would put before honour was his own family, as seen by the fact that he confessed to wanting the throne for himself (bull****) because it meant sparing his daughters. Brandon Stark was more of a hotheaded, strong willed, and albeit,more selfish Stark. It would been more of a Brandon move to see bastardy as something you can't rise above, not Ned, because Ned made the Stark family into a unit that accepted anyone who adhered to his own values of loyalty and duty to family. He was nothing like the other Starks like his father, or Brandon, because he played by his own rules, his conscience, and his love for his family (i.e., letting Arya keep her sword, finding her a proper teacher for her sword!!! unheard of in those times, except for only a few families like House Tarth and House Mormont ) rather than anything anybody else thought was "proper" and "right".
Part of me has to wonder if that's why Catelyn loved Robb so much, she seemed to see a lot of Brandon in him rather than Ned, and she also hated Jon for looking so much like Ned.