Characterisation is just hard, in general. I think it's difficult not to write idealised versions of characters, particularly since fic generally comes out of love (or at least liking). Which might be another reason why cheating doesn't get dealt with very much. It's unpleasant to think of someone you really like as being a dick.

(Oscillating between excitement and terror and deep, deep loathing for the bigbang atm. Only halfway to the wordcount D:)