I so don't get Marriage Law. I mean, at least Severitus (for another example of ones of those perennial challenges) is one of those things you could shoehorn into the canon (especially pre-DH) because of the part physical resemblance between Harry and Severus and the way that the time around Harry's conception is all a big mysterious hole in the canon. (I actually don't care for Severitus, it was just a well-known example.)

I am not a canon nazi when it comes to fanfic; I know most of it is non-canon by nature. I don't mind "butterfly effect" AUs, either (in fact one of my favourite fics is sindie's "The Moment It Began", which is a huge epic in that genre) . But I at least want plausibility. Something really contrived is all right as a humourous device and I don't mind so much in PWPs, but when done For Serious, I usually find myself sighing and shaking my head.