Coriolanus’s classism shines through here. He believes he must preserve his own family’s fortune and reputation as much as possible, and he doesn’t feel that anyone else—like the Plinths—should be able to buy their way into the Capitol’s high society circle. But hating the Plinths is also personal, as they’re profiting in the same line of business as the Snow family once did. This also drives home how devastating the war was for everyone, including the wealthy. By ignoring Sejanus (rather than, say, bullying him), Coriolanus can show Sejanus he’s unwelcome—while also making himself look good, since he’s not overtly being mean to Sejanus.