Let her talk, and when she's done, steer her back to homework. Imagine the worst now, before your kids are actively dating, and talk about it. You want your teenager to learn that later on he can come to you with any problem, no matter how sticky. Don't tell your teen how to handle relationships. Ask her for her thoughts.
Teens are philosophical and appreciate being asked for their viewpoint.
You'll probably learn that everything you've been teaching her for years has actually taken root and flowered beautifully. Tell love stories. Tell how you and their dad met, how your parents met, how your grandparents met, and go ahead and be mushy about it. This is the stuff your teen is trying to figure out how to do, and he'll be riveted. Even if you are now divorced, bury the hatchet and tell stories of when you and your spouse liked each other.