Harry never asks her if she's 'experienced'. He just assumes that it's her first time, like it is for him. She suspects he likes it that way, likes the idea that they're both learning something new, that they are equal.
She doesn't tell him about Tom, because she knows that would upset him, and anyway she doesn't want to talk about it. She doesn't even remember it, really, it's all sort of fuzzy and blurred.
She doesn't tell him about Draco, because that would upset him too, perhaps even more, because it was her choice, her shy smiles and her breathless "we can go to my dorm, there's no one there at the moment", her nod when he asked, "is this okay?"
She doesn't tell him about Hermione, because he'd almost certainly tell Ron, and that would cause any number of problems for all of them. She and Hermione have a silent agreement not to tell anyone, anyway, and it didn't really mean anything, just two friends on a hot summer's night suddenly curious about what would happen if they followed through on their giggly jokes about how girls are probably much better in bed than boys.
She doesn't tell him, and she's glad he doesn't ask, because this way it feels like this time, this night with the boy she's loved for as long as she can remember, is her first.