Buffy laughed.

"Kids aren't like cars. You don't get to trade them in when they get too old to handle. Sometimes, you can't even pick at what age you get stuck with them."

Remember? After all, their 'marriage' had come on the back of an unexpected familyhood. Even Buffy's more authentic experiences jived with her philosophy, as she'd been forced to step into guardianhood shoes with a very volatile teenaged Dawn.