Harry doesn't recall how he ends up responding exactly to Draco's question, just remembers  wanting  and  yes, god, yes, which he apparently manages to convey well enough for Draco to take him by the hand and lead him inside. She pulls him through the kitchen to snag a bottle of wine from the counter, flips off Pansy once more when they pass her, who returns the gesture even though she is making out with Ginny again, and then continue up the stairs.