Then you completely missed the point.
Step 1. Robb has a big victory, takes the Crag. He is well on his way to chewing off a big chunk of Lannisters' own lands. The victory might have come a bit easier than it should have.
Step 2. Robb is injured in the battle and bedridden.
Step 3. The Westerlings (on orders from Tywin) encourage their daughter to befriend Robb and help nurse him back to health. They may or may not have suggested to her that their best chance for survival was for her to "comfort" him when he is at his most distraught after receiving news that his brothers are dead.
Step 4. Honorable Robb realizes what he has done and insists on marriage (or is goaded into it by her parents with some subtle digs at his honor) rather than be seen to have taken advantage of the girl, who was essentially his prisoner at the time.
Step 5. Tywin is informed and sends word to begin negotiations with the Freys before Catelyn can begin trying to contain the damage.
Step 6. Profit. Also a lot of blood.