It's sort of queasy to imagine Luke Skywalker having sex. Han Solo, certainly, the man must've gotten around. Lando Calrissian, even moreso. Leia... well, Leia had the bad fortune to look like Carrie Fisher which is no doubt prejudicing me here, but it doesn't disturb or surprise me to imagine Leia having sex.
Darth Vader (original trilogy Darth Vader, in costume) is sexy as hell.
But Luke Skywalker can't have sex because he seems like a symbolic child. Not just that he's boyish but that he's too closely entwined with my own childhood. I watched Star Wars for the first time when I was very young and I identified with Luke. Now when I rewatch it (which, despite how this entry sounds, isn't that often, I swear) I don't see a twentyish space adventurer--I see a five-year-old Holly.
Halfway related: I have a pair of stuffed animals, a horse and a cow, that I've had since birth. I've always slept with them in my bed; when I was little they kept away monsters. But I've developed a sort of superstition that Horsie and Cowie must never "see" me having sex or masturbating. They have to go under a pillow or into a drawer every time.
Nearly everyone involved with BDSM is a huge geek. The reasons why deserve their own entry. But in case you weren't aware: the correspondence between BDSM enthusiasts and sci-fi/fantasy/Ren Faire/anime/"graphic novel" enthusiasts is essentially one to one. The debauched, naked, bruised-ass underbelly really digs on Firefly.