What is Firefly?
People have varying reactions to varying degrees of sexuality. Some prefer not to speak of it, others embrace a larger view, not just the narrow, dogmatic views of the conservatives. Firefly has enough sexuality to satiate our more amorous fellows, while keeping everything tasteful and purposeful, enough so that our conservative friends can still enjoy the show, even with their mothers watching.
The 'Verse has an interesting idea of sexuality, and holds it in high regard, as demonstrated by Companions or, for the heretics present, space-whores. Companions are trained to be exactly that, companions. They are perfect in every way, masters at seduction, intrigue, sword fighting, and they know how to give a guy a good time. Companions are trained in a formal academy, located on the planet Sihnon, home of the Companion's Guild. Girls and boys are accepted at young ages and trained until they reach adulthood, not unlike our familiar Jedi. Like Jedi, Companions perform a valuable service, and are discouraged from attachment. Jedi know the ways of the Force, Companions try to feel the energy of a person, or, as Cap'n Mal would put it, their wallet, it has a kind of, aura, about it. Of course, unlike Jedi, Companions charge for their services, and they don't come cheap.
Serenity has her very own Companion, Inara Serra. Mal finds that having a Companion on board opens certain doors that would otherwise be closed to him, plenty of planets won't even let you land without a decent Companion on board. Inara occupies a shuttle, where she entertains all kinds o' fancy folk, men and women. You could ask Jayne, but he's in his bunk. It's important to remember that Companions are very important people, at the top of society, respectable. As for the poor, don't think a one of 'em ever seen a bona fide Companion before, and they're likely not to. Inara also has feelings for Mal, and he for her, but they won't be talkin' 'bout that anytime soon.
What about the Big Damn Heroes?
Mal and Inara have an unspoken attraction for each other, why they refuse to act on it, I'm not sure why, but it's there, as demonstrated in certain episodes, at times more clearly than others. Nobody's taken hold of his plow for a while, mostly because he knows that kissin' girls makes folk sleepy. Zoe is married to Wash, and hopes to have a child with him one day. As for Wash, "Everybody not talking about sex, in here. Everyone else, out there." Jayne knows what he likes. Simon is a bit in cryo himself, Book is a preacher, River is crazy, and Kaylee is, boy-crazy, I think the kind term for it is.
I have heard, on occasion that some people didn't like certain sexual aspects of Firefly and Serenity, perhaps that some of it was unnecessary. This is not true. Most complaints are about one of two topics, either that some of the sexuality with Kaylee wasn't needed, or even that the sexuality altogether wasn't needed. As far as Kaylee goes, it's part of who she is, and it's certainly not thrown around in abundance. Mostly I hear this one relating to the movie, Serenity, in which Kaylee utters a certain line, which is quite funny, I might add. If you can't handle ten seconds of sexual humor, I might have to inquire as to where you keep your sense of humor, 'cause I want it, you're obviously not using it. I have also been told that some people feel that there is too much sexuality in Firefly, which is simply ridiculous. Firefly is supposed to be the real story of real people, not the movers and shakers, not the heroes, not the Chosen One, just folk doin' what folk do, and sometimes, in life, sex comes up. How far does it go? A space hooker, and one episode with a whorehouse. It's a western, after all.
No matter whom you are, or what you believe in, you can still enjoy Firefly. If you don't, there's a good chance Reavers will get you. Rape you to death, eat your flesh, and sew your skin into their clothing. Never said it was pretty.