I suppose it's just a matter of personal vision for the main book, as in what you want out of it.
If the main book features a single Spider-Man, then a married Spider-Man would automatically be AU or a prequel taking place before OMD. If the main book features a married Spider-Man, then vice versa.
I realize me not caring about the marriage is an unpopular opinion on online circles but whatever.
Well, It wouldnt too hard to write. MC2 Peter and MJ have essentially the same history as 616 up until Clone Saga.
Also remember if it was solely about the character's progress through life, then Peter would grow old and die. I guess I just don't care much for the marriage part of the character pprogress. Nowadays, many people don't have a strong desire to marry and have kids, birth rates are below replacement rate in many countries.