Hermione isn’t upset, as the boys had assumed she would be. She understands their hesitation in telling her—she’s not exactly out and proud about their... "thing’. It was also a little misogynistic, "staking a claim’, like she’s a piece of property. But in their defence—strangely chivalrous? She doesn’t understand pureblood dating customs. But the conversation had answered some unasked question about their exclusivity.