Your explanation for Snape's reason for the affair is probably the same reason that many people have affairs. Too bad Harry's love and desire for Snape, or his love for Harry wasn't enough to keep Severus faithful or happy with only Harry, which again is very similar to what happens to people.
I think I saw you mention in another comment that Snape viewed himself as Harry's Protector and didn't want to hurt him. I guess I find Snape's reasoning difficult to believe if he creates a situation that actually hurts Harry. Snape somehow thinks that protecting Harry is involved with physical comfort and protection, and not emotional.
Maybe you could write a fic where Harry gets his revenge and puts Snape in his place and that Snape knows he deserves it, and is willing to eat humble pie no matter how long he has to eat it ;P I would find that very satisfying ^_^
It's always interesting to understand a writer's reasoning or interpretation of a character's motivation or behavior. Sometimes it's quite clear, and at other times it takes longer for me to understand what the writer is trying to convey :)