In the season one episode Woman on the Verge, Chuck mentions that he lost his virginity to troublemaker Georgina Chapman in the sixth grade.

What happened Georgina Sparks?

During the first and second seasons, Georgina played a minor role but she returned for longer periods for seasons three through six. During season four she gives birth to her son, Milo Humphrey, and marries Philip Becker. In the series finale, New York, I Love You XOXO, its revealed she has begun dating Jack Bass.

Does Dan cheat on Serena?

In A Thin Line Between Chuck and Nate, Dan finds out via a Gossip Girl blast that Serena thinks shes pregnant. Shes rescued by Blair, Nate, and Chuck, who try to keep Dan away from the situation. To keep Dan from knowing what she really did, Serena lies that she cheated on him and he tells her that he is done.