His marriage to her makes more sense in the book- he's doing what he believes that Ned would have done. If Robb had preggo'd Jeyne and not married her, the kid would have been a bastard. Having grown up in the same household as Jon Snow, Robb knew the crap bastards often had to go through (coughCatelynTullycough) and didn't want his possible child to go through with that. Plus he had already soiled her, which would have made it harder to arrange a good marriage for her. Book Robb was doing what he believed to be the honorable thing, while show Robb was just thinking with his crotch. Though I have to say that the show gives the viewer a better idea of why Roose Bolton turned on Robb.