rob need the frey's forces since the carstarks left, however he was suppose to marry his daughter after the war, as the price for allowing rob to cross his bridge. rod reneged on the deal by marrying that girl, so in that scene he was negotiating with frey's sons a marriage between rob's uncle and a frey daughter. not wanting to be fucked out of a deal again, frey's sons demand the wedding happen asap. rob's uncle is pissed, cuz frey's daughters are notoriously plain/ugly.
Okay I'm tracking. I'm caught up on that. It was whatever Lord Edmure did is what I'm not getting. I gather his marriage is suppose to make up for his shenanigans. What did he do wrong?