I never really emphasized with Peter Parker, married guy. I started reading when I was 12 or so, and it was the most boring aspect of the character. Even though the character's name is Spider-Man, in continuity he's never going to be older than the hyperactive, slightly manchildish, mid 20's guy - a character whose trait is kinetic hyperactivity. Settling down and worrying about mortgages? So doesn't reflect that.
And then take into account at least in comics, we're talking about a medium with a possibility for infinite stories with no "end" date, that also thrives on creating drama for it's characters. How much drama can you bring into a marriage? Oh no, Mary Jane is smoking! Oh no, Mary Jane is out all night partying! Oh no, blah blah babycakes. It got monotonous.
But the key reason to keep Parker single is because of the internet and it's penchant for breeding crazy people who love to ship fictional characters together. A world without DarthFury finding ways to put Spider-Man and Psylocke together is a word I do not want to live in~