What really made Spidey unique wasn't so much his powers or his costume, sure those were cool things, but what really made him unique was that it was about the guy inside the costume and the soap opera that was his life. Peter could have had a whole different set of powers and it still would have been a ground breaking comic because in the end, that's not what made Spider-Man stories different. So, with every little bit of the trappings of his life that got chipped away, more and more of the soap opera dwindled.
When Peter Parker got married, it caused the character to be cut off from many of the social situations and settings that put him at conflict with his family, friends, and especially the girl he was dating. Suddenly, something as simple as the tension he had with Felicia Hardy was completely defused; if Peter ever gave in to temptation or even considered it, he would be, in the eyes of the fans, the lousiest guy in the world. It became harder to place Peter in situations where he could hang out with other single characters, without him seeming like the oldest person in the room, even if he wasn't. And whatever nerdish sex appeal he possessed, we had to tread very carefully. He became the perpetual "designated driver." Sure, Peter could hang around with other married folk -- I bet that would be exciting!
Let me try to put this as plainly as I can, and let's be really honest here, let's really look at marriage for a second. I'll get personal, for a moment. I have an incredible marriage and a fantastic kid, but there is no question that my life was much more story-worthy when I was single. Was I happier? Absolutely not. Was my life a better story from a drama sense? Ummmm, yeah. It had many more twists and turns and theater and was a bit of a mess. Now let me say, not everyone, but for most: When people get married, they tend to settle down -- life slows down and you gain different responsibilities, grown-up responsibilities, boring responsibilities. You go out to dinner less, see fewer movies, your social life is curtailed and revolves, as it should, around your significant other. In short, life hands you a mini van. While marriage makes for an okay story, there is less drama in a (healthy) marriage than in a single relationship. That's one of the many reason we get married -- we want stability, we want comfort, we want kids, etc., etc. No one gets married because they want more drama in their life. What's good for one's life doesn't always make for great stories when the heart of your character's universe is drama. From a writer and artist's point of view, the people who are creating the stories, it's like giving Daredevil his eyesight back. It works for a short time and eventually erodes at the foundation of the character and what makes them unique. We all want Peter to catch a break and to settle down and have happiness in his life, but that isn't really what we want. If that actually happened, people would stop caring about Spider-Man.
Bottom line, there are so many things that twentysomethings are doing with their lives that a married Peter can't. He needs to be a single guy. Sure, he can have a girlfriend -- that adds something to his story -- but a married Peter just cuts off too many avenues for good soap opera. Could you have soap opera within a marriage? Sure. But after a while, there's only so much tension you can bring into Peter and MJ's marriage before you make him seem like a louse of a husband, or her, like a bickering wife. In contrast, you can only play them as a happy-go-lucky couple for so long -- that adds up to zero tension within the relationship and takes away a crucial element of Spider-Man stories: the soap opera.
