Well fuck, Joe Quesada had it in for poor old Mary Jane from the moment he took the helm at Marvel. The writing was on the wall from the beginning: married Spider-Man was a bad idea that needed to be done away with, but no good answers presented themselves. They toyed with a few different ideas through the early part of the decade - separating the couple, introducing other love interests, killing Mary Jane outright - before realizing that any kind of drastic measure would saddle the character with as much or more baggage as what they were trying to jettison in the first place.
As flip as Quesada's responses over the years were, he was essentially right: the only thing worse than a married Spider-Man, in terms of clearing an accrual of baggage from the character, would be a divorced Spider-Man or a widower Spider-Man. Aquaman can get away with being divorced, the Atom can get away with being divorced, but neither character's domestic drama is necessarily the focus of their appeal (which are, respectively, underwater shit and shrinking). But you will notice that the introduction of marital woes pretty much irrevocably changed both characters, in that both of their sagas took an immediate and irrevocable turn for the dark. Oddly enough, neither characters' status quo was reset by the original Crisis, which seems in retrospect like a misstep to me. It is possible to make good Aquaman stories where Aquaman is divorced, because the strip's focus has always been the character's milieu - but still, once you give him marital difficulties, they will weigh him down like an albatross for the rest of his existence. He really can't be "fun" like he was back in Super-Friends days because divorce (and a dead child!) are irrevocable signs that someone is Getting Older. From that perspective, I can see the appeal of just shuffling the old guy offstage and replacing him with a younger, hunkier version with a sword - one without an ounce of baggage.
But Spider-Man is different. Personal drama isn't just another element of his mythos, it's inarguably his main appeal. Spider-Man works best, and always has, when he's at the center of an overheated soap-opera. Marrying the character changes the focus of the soap-opera. Conversely, The Fantastic Four is not really a soap-opera, despite the book's soap-opera elements: Reed and Sue have been married long enough and from such an early date that the marriage is and has always been the status quo - a stable marriage isn't really boring in the context of the Fantastic Four because the book presents a stable nuclear family model as the backdrop for its cosmic adventures. That's the book's premise in as succinct a manner as possible. But Spider-Man, he's the "Hard-Luck Hero". He has to be having some kind of personal trouble or he's just another generic long-underwear type. And personal trouble that happens to married people is of a different type than what happens to single people. It's a lot harder for a general audience to relate to, for the exact same reasons. In movies and TV, young people's relationship problems is usually the basis for comedies, whereas married people's problems are usually the basis for dramas. It's not impossible to create serious dramas about young love, or comedies about divorce - but the former often carries some kind of campy or trashy subtext (The OC), while the latter is usually regarded as mordant or macabre. Certainly, it is impossible to imagine Amazing Spider-Man turning into a black comedy about adultery and divorce.
Spider-Man has been married for exactly twenty years, 4/9ths of the character's existence. There have probably been just as many stories written with a married Spider-Man as not. Many generations of comics readers have grown up with a married Spider-Man as the default (Ultimate version notwithstanding). Some of my favorite Spider-Man stories (for nostalgia's sake, if not on a strict aesthetic basis) have been married Spider-Man stories. I think Sal Buscema's married Mary Jane from his long Spectacular run is one of the sexiest comic book females ever. But writing good married Spider-Man stories has gotten progressively harder. How do you keep the character interesting without darkening the tone of the books to a regrettable degree? From that perspective, it isn't hard to see why the Clone Saga happened: they needed to do something to shake the character up, and with his personal life in forced stasis, they could only impose plausible status quo threats from external forces. When J. Michael Straczynski delved into all that bullshit about Spider-Totems and mystical junk, he was taking the franchise in an unpleasant leftward direction that worked against its strengths, but with Spider-Man's personal life - the traditional engine of his best stories - essentially no-man's land, it was probably the best he could do with the materials he had to work with.
I had a brief conversation with a friend of mine recently about Spider-Man, the thrust of which was essentially that dark and gritty Spider-Man stories were completely besides the point. My friend reminded me that regardless of whether or not these stories make "sense" from a characterization point of view, they don't make sense on the more basic level that Spider-Man isn't really a character. Sure, you can argue that he has character traits, but ultimately he's not a dynamic creation, and hasn't been since around 1967. He is a property, a static archetype to be plugged into any number of different types of stories. While the comics industry has been trending darker in its storytelling content for at least 30 years, the trend accelerated in the late 1980s and the effects have been especially harmful to Spider-Man. The books have been cycling for years, with stories getting progressively darker, only to be brought up short by a "back to basics" correction, shortly after which the downward trend resumes. Is there any wonder Marvel wants to put the marriage back in the box?
So now we've got "One More Day", a storyline which promises to almost certainly undo the marriage, as well as Spider-Man's recent unmasking. Honestly, the unmasking was probably the first clue that Marvel had finally gotten the guts to call a redo on the marriage, since such a drastic change to the basic mythos practically necessitated the advent of a cosmic do-over somewhere down the line. I have a great deal of affection for married Spider-Man, but honestly, when I examine the problem from all angles, getting rid of it is a sensible decision. For whatever reasons - lack of imagination, creative entropy, overall industry trends - married Spider-Man just doesn't "work" in many important ways that Spider-Man needs to be able to work in order for the property to remain viable for the broadest and potentially youngest audiences. Marvel has always avoided drastic Crisis-style reboots, with good reason, but from the looks of things they are about to break that rule in a big way.
To me, Spider-Man will always be married. Marvel does not publish Spider-Man comics for me, however, nor should they - for one thing, I haven't actually purchased a Spider-Man comic in years. The property's potency should depend on its appeal to new generations of readers, not the nostalgic memories of longtime fans. But the fantasy of a joyously married Spider-Man swinging into the sunset for one last happy ending is tremendously appealing to me nonetheless. When I was younger I accepted that Spider-Man was married without any problem - sure, it was a departure, but it made sense at the time. It doesn't make much sense anymore. But I'm much older now than I was then, married and since divorced myself, and a lot of things that used to be certainties are no longer as they were. I don't look to Spider-Man for any kind of affirmation in my life, but I feel sympathy for Peter Parker in this instance nonetheless. All things eventually pass. Only in comic books do we get the luxury of do-overs, even if something vital is still inevitably lost.