They know each other’s bodies inside out, and have since childhood. Seppius couldn’t even count how many times he’s shooed Seppia’s slaves out so he could be the one to help her ready herself for bed; the one to spread her glorious hair across her bare shoulders.
He helped her ready herself in other ways as well: assisted her with loving touches as she discovered the pleasures of her own body – and later, his.
He’s always known it couldn’t last but when he sees her with Varinius, the strength of his urge to murder him still surprises Seppius. She’s his.