Because a female character's sexuality shouldn't be organic to the story, it should be a gimmick to titillate viewers.
From Contact Music:
HAYDEN PANETTIERE is preparing for girl-on-girl love scenes in the next series of HEROES - her character will reportedly enjoy a smooch with her college roommate.
The 19-year-old actress plays cheerleader Claire Bennet in the hit U.S. show and TV bosses are considering working a lesbian storyline into the upcoming fourth season.
A source tells British newspaper the Daily Star, "It's just girlie fun at first. But it might progress into something more serious. It depends on how viewers respond."
That's right – Claire's lesbianism/bisexuality is such an intregal part of the storyline, we'll let the viewers decide. If they vote "HOT!", there will be more.
And sexual experimentation with a college roommate – that's BOLD! Plus: cheerleader!
Don't get me wrong, I'm all for having more LGBT characters in sci-fi entertainment, across the board. And I think, overall, it's better to have more related characters and storylines than less, even if I think some are either promoted poorly or used purposely as a gimmick.
But I think the reason storylines like Willow becoming gay/bisexual worked for me is that they didn't issue a press release when it happened. It was just part of the story, and I don't feel it was used as a gimmick. If anything, BTVS was at the height of its popularity when Willow and Tara got together, and could have lost (and probably did lose) some viewers in the process.
That's just my 2 cents. I could be wrong. It just would be nice to have a high-profile gay character in a comic or TV show and not promote it as "TEH GAY CHARACTER OMG!!!!!!"