"Hercules: the Legendary Journeys." The first few episodes are in the form of mini-movies, and both Hercules and Iolas get married and have children. When the series proper begins, Hercules' family are killed, so they have an excuse for disappearing, but Iolas' family just sort of evaporates and are never heard from again.

Could someone edit the Angel example for chatter? It has spoilers so I can't do it as I haven't seen the final season.

Also, I'd ask whether or not to remove the Merlin example altogether - it doesn't fit the trope to me, but what do you think?