If you think about it, Spider-Man actually got pretty lucky with how his powers turned out.  If you think about what an actual hybrid spider and man might look like, it's a pretty freaky image.  Something tells me old MJ wouldn't want to be making out with him in the rain as he hung upside down if he had multiple eyes and legs and fangs.  Unless she had a really weird thing for spiders or something.  It takes all kinds, after all.