They both are looking at each other intently, it's painfully obvious that they're a couple, sejanus kisses coriolanus on the cheeks every minute or so, they notice your stare

Sejanus: Hey, wanna come sit with us?

Coriolanus: Come on, sit with us.

You look at the empty seat next to them, will you sit with Sejanus, or Coriolanus... if you even want to sit with them at all