I agree with those who say Sci Fi is a sub-genre of Fantasy.
As for why do women prefer fantasy over sci fi? Damn, I really don't know, but I too have noticed that. Normally I am really quick to speculate on the inner workings of female behavior, but I got nothing.
As for the success of Twilight; this has nothing to do with the appeal of "fantasy" and everything to do with the seduction of sexual metaphors and hot guys with washboard abs.
It is my belief that Twilight (and similar movies and shows) appeal mostly to girls between the ages of 11 and 14, when they are beginning to sexually mature. They may not have "true" sexual desires at these ages, but certainly their hormones and neurology are preparing them for it, and these metaphors are stimulating their repressed or under developed sexual needs.
Let's face it, Twilight isn't about evil scary vampires and scary werewolves, it's about super hot sexy young super men who are charismatic and cliched.
While I have only caught glimpses of Twilight from my mother and neice watching the films, I can already spot the cliche of a "dangerous guy I shouldn't be attracted to, but are." and "nice safe guy who loves me, but I don't feel the same way".
Furthermore, vampirism in and of itself is a sexual metaphor. The sexy vampire sinks his teeth into you for the first time (you've lost your virginity). Any time a creature has a compulsive, lustful, need to transform you into one of them, you are losing your virginity upon transformation (metaphorically).
The compulsive lustful need to drink blood is symbolic of wanting to get laid.
Twilight is a soft-core porn and romance that just slaps the label "vampire" and "werewolf" on the actors.
The appeal of Twilight does not reflect the appeal of fantasy.
The difference between sci-fi and fantasy is strictly aesthetic.
Fantasy: picture a knight, on a horse, with a 15th century English castle in the background. There are elves by his side firing arrows at an oncoming crowd of knights and elves.
Sci Fi: Picture a goofy dressed warrior with a laser gun, a star ship in the background, a bunch of aliens by his side, firing lasers onto a crowd of other aliens.
Cowboys vs Indians, cops vs robbers, humans vs aliens, humans vs orcs, allies vs axis, British navy vs pirates, Yankees vs confederates. It's all the same game, different aesthetics.
As for the notion that science fiction is "scientific" oh rubbish!
Let's compare star trek to harry potter.
1.Harry Potter uses a teleportation stone that works on "magic",
Scotty uses a teleportation device that works on "science".
2. Harry Potter uses his wand to shoot a "magic" bolt of energy at someone.
Kirk uses his phaser gun to shoot a "scientific" bolt of energy at someone.
3. Harry Potter uses a magic spell to make a roast appear out of thin air, in his bag.
Kirk uses a scientific food replicator to make a roast appear out of thin air, on his table.
4. Harry Potter uses this "magic" spell to get out of a tight jam "Confundus Expecto Patronum Stupefy."
B'Elanna Torres uses this "technobabble" to get out of a tough jam "Run the plasma through the tachyon dispersal matrix into the neutrino flux regulator."
5. In some episode of Harry Potter, Hermione used a magic spell to create a force field around them to keep them from being seen by their enemies.
On some episode of Voyager they used some sort of tachyon arm band to avoid being scanned for by their enemy.
There is nothing scientific about science fiction.
Fantasy is sci-fi in a medieval period, sci-fi is fantasy in the future. Both can be utopian or dystopian, and all of it is cops and robbers.