I know I've said this before, but I think it bears repeating. I believe the real reason why various creators at Marvel over the years were against the idea of Spider-Man being married is because it made them realize that Spider-Man was no longer a kid but a full-fledged adult just like every other superhero. Which, of course, goes against the very image of what Marvel has tried to promote Spider-Man as--a teenage and/or young superhero who's not a sidekick to another superhero.
Of course, I'm of the opinion that it wasn't Peter's marriage to Mary Jane which made him an adult, because he was already an adult by the time that happened. As Colin Smith of Too Busy Thinking About My Comics and other posters have pointed out, Peter getting married to Mary Jane at that point in time "wasn't a betrayal of the character. It was the logical, even necessary culmination of Peter Parker's life-story."
No, I believe the real point in which Peter became an adult wasn't due to Jim Shooter--it was because of Marv Wolfman. That's because Wolfman did two things to Spider-Man during his run on Amazing. The first was to have Peter graduate from College, which, like it or not, is still universally regarded as passage into an mature adulthood, where you're expected to start "acting your age." The second is when he had Peter involved in what as, for all intents and purposes, an adulterous affair with a then married Betty Brant. Granted, it wasn't explicit and was rather tame by today's standards, there's no getting around the fact that Peter was dating (and hinted to have slept with) a woman who, while separated from her husband, was still legally married. And if the argument is that Spider-Man is supposed to be about "youth," then those two plot developments are the moments in which Spider-Man crossed a line that he could never go back across no matter how hard Marvel has tried to do otherwise.