Thank you so much for all your comments throughout the past months! I can understand why the ending wasn't what you'd hoped, but that was how we felt it had to be in order to be true to the characters as we'd developed them.
For Remus and Teddy, it was teaching/learning, and although yes, a secret from their partners, I don't think either of them saw it as a betrayal in quite the way you read it, since although there were some unspoken assumptions, neither Remus/Sam nor Teddy/Andrew had made any overt commitments to be exclusive. Perhaps someday Teddy will tell Andrew that he had in fact done more than he admitted originally, although I doubt he will say who his partner was.
I'm glad you liked the story as a whole even though you'd hoped it would end differently!