It is entirely possible that they had sex for the first time after the Millennium kiss.
It is also possible that they went home together to her place or to his, and just made out until they were too tired to keep going. Smiling into kisses, hands roaming, over each other's hair, shoulders, backs. Pulling back to look at each other, just look, eyes full of awe and gratitude and happiness. And . . . love? Quickly diving back in for another kiss because that is a big word, such a big word, and too much too soon, but unsaid is not unfelt.
Something has changed, but not changed; it's not any different than it was before. It's not more than it was before. It's just the difference between knowing and knowing. Between feeling something and putting a name to it. The ground doesn't shift, there is no earthquake, no lightning, no fireworks except the ones outside their window. There is only them.
Only them, hands finding each other, his fingers soft against her cheek, her palm warm against the back of his neck. Time and space can do without them for the night, the world is turning like it always has, and they're sharing each other's warmth, breathing each other's air. Adding vocabulary to the silent language they've been speaking to each other for years: touch. Now more than his hand on her back or her shoulder against his. There is so much to say, and they are saying it, not all of it, but enough for now; through touch, through looks. It's not a new story, just a new chapter. They fall asleep before they can add the next one, but there's time. The world didn't end, after all.