It's an ongoing joke that fans of anything geeky (D&D, comic books, video games, sci-fi, runes, mythology, The History Channel) lacks a love life. You know the stereotype. So, is it coincidence that some of the very best couples are found in the geek genre? Are they trying to let all those lonely nerds get a glimpse of the good life? Or is it just that uncertain futures, alternate universes, galaxies far far away, or splash page bound stories lend themselves to bigger and gutsier stories? After all, sci-fi, fantasy, and superhero stories tend to break gender barriers in all sorts of ways. Women were packing superpowers and weapons in genre stories a lot sooner than they were on big or small screens.
As to why that is, maybe it has something to do with the origin of sci-fi, fantasy, and comic books. Sci-fi, comics, and fantasy inherited the old mythologies of Homer and King Arthur, where people expected great highs and lows to everything their heroes and heroines did. If lovers weren't tragically separated or punished for their sins, and there wasn't a lot of sword fighting and slaughter in between, then everyone was bored. Nowadays, you can't have heroes who are gone for decades and battle trolls unless you're in a particular section of Blockbuster or Barnes and Noble, because it's just not thought to be very serious stuff. (Unless you're a literature professor. Then you know better.)