The unspoken reason Robb marries Jeyne Westerling after they have sex is because he doesn’t want to be like his father, right? He doesn’t want to knock someone up in the middle of a war and wind up with a bastard, making no one happy — not the dad, not the mom, not the kid, not the Frey wife, not the Frey wife’s legitimate kids.
Robb loses the war he’s fighting to avenge his father because he doesn’t want to be like his father.