I have always believed this and they were put up to it by Tywin Lannister, to screw up the Stark/Frey alliance.
Seems far fetched. I know it's in the west and he has spies, but Tywin was able to get a message to people inside a castle captured by his biggest enemy in a matter of days? He didn't even know Robb had moved west.
Remember Robb is a horny 16 year old. The Freys are known for being ugly and he nearly died in battle. Makes sense he would want to sleep with a prettygirl (his nurse no less) before he might die. Likewise Jeyne is about the same age and Robb is a handsome king. Two horny kids.
Also this theory ties in much better narratively- Robb's sense of honor ruins him much the same way it did his father.
This post was edited on 12/9 at 10:09 am