I thought it was realistic that those two, the most sexual characters, had been together in the past (and I'll never understand why it wasn't established earlier in the show ... Possibly because they thought Haley wouldn't go near him if she knew
) but I think leaving it until season 4 to reveal it, the point where Haley was pregnant... The timing was completely off and oh dear the attempts to make the actors look like they were doing scenes from 4 years ago
pretty bad
you could tell their looks had changed - hair, bodies etc.
Relating this to Harry Potter, I used the Nathan/Brooke example to show that you don't need to have a central relationship to the story to have a fanbase. People can ship all types of AU pairings based on their own ideas of what they want for the characters outside of the canon.
This applies big time to Draco/Hermione.