That’s how I interpreted the scene. She was “using” the mechanic the get access to a Firefly class spaceship (and enjoying a little sex while she was at it). Seing that she 1) is fascinated with engines, and 2) openly enjoys sex, hooking up with the mechanic, even if he was kind of a moron, was a pretty sweet deal for Kaylee.
In other words, I agree with DianaG. 
Ah! Wait a minute! I think I know where you’re coming from now. It’s not that it’s out of character for Kaylee so much as it’s out of character for Hollywood Writers, yes?
Are you new to Joss Whedon? He does stuff like this all the time. He’s practially made his living on taking standard Hollywood cliches and turning them on their heads. The vulnerable young girl becomes the vampire slayer (or the vampire, in Darla’s case), the talkative henchman gets kicked into a turbine, the sweet nerdy girl brings marshmallows to a pile of burning demons, the loyal sidekick and “safe” series regular dies for good in the first season and doesn’t even return in flashback, the “captain has to do this for himself” moment turns into a cry for help, the “genius mechanic” - surprise! isn’t the genius mechanic we’ve known about all along. Instead he’s shagging the sweet young thing who’s more genius than he is.
I think, at it’s core, it’s part of Joss’s humor. He likes to surprise us by having his characters be and do things you don’t expect if you’re well versed in Hollywood stereotypes and techniques.
Plus, he is smoking hot.
I’m not exactly new to Joss, but nor am I particularly well versed. I used to watch Buffy a lot, first couple of series, but as often happens to me, I miss an episode, or half a series, and suddenly I can’t watch it anymore because I don’t know who the new characters are, or the ongoing storyline.
So yeah, it is kind of that surprise at the character not being what you expect. But I do expect if from Joss… I mean, (SPOILER!) the alliance guy being shot by Mal at the end of the pilot. It’s a tense build up, obviously going to be some kind of stand off here, wonder how it’ll pan out… BANG! Oh. It’s over - didn’t expect that. And the henchman through the engine too - the Hollywood-ised brain is thinking “ah, here is a nemesis who will come back in later episodes”… nope!
All of that didn’t bother me… because it was a means to an end - a plot twist. But the Kaylee flashback didn’t seem to go anywhere. If it had been a surprise, but then later episodes had built on it somehow, I wouldn’t have started this thread. It just seemed to go nowhere. Perhaps it’s all part of this longer character development silenus has pointed out we didn’t get. And Troy’s right - part of the confusion is that this guy is a loser.
So now I’m thinking “alright, she heard about the engine, came on to this guy (who happens to be attractive ;)), and used him to get to the engine”. And it turned out better than she expected coz she ended up working with it on a permanent basis. This all fits a bit better with how I imagined Kaylee to be.
And the rest of you are all thinking “well, durrr!” 
Well, in your defense, my initial gut reaction to the scene, before my brain really had a chance to process it was: “Whu? Kaylee’s a slut?”
I think it’s a product of my Hollywood-ized brain, as you put it. When you see a women depicted the way Kaylee is during her introduction, it’s usually movie shorthand for “she’s a slut”. We rarely see scenes of women enjoying casual, non-relationship sex without it being a means of painting her as a Woman of Loose Morals.
So it’s actually starting to make a lot of sense to me that this is yet another example of Joss turning a movie convention on its head for shock/humor value.
There’s not really a lot of opportunity for Kaylee to do any boinking on Serenity. Look at her options:
Mal: The captain, and too much drama between him and Inara to get between.
Wash: Taken.
Jayne: No thanks!
Book: Celibate.
Simon: The best option, but he’s too shy and socially inept for this to proceed quickly.
I think if the show had developed Kaylee and Simon would have eventually gotten together, or she would have found another outlet for her lust.
You mean, another outlet what doesn’t run on batteries?
And I really don’t understand what Simon’s problem was. The most perfect girl in his world? Heck, probably one of the most perfect in the 'Verse! is coming onto him. He wants to play it slow because, from his own words, he wants to make sure to follow all the societal forms and get it right, since that’s the only way he knows to show respect to a woman. Well, heck, there’s a preacher right on board! What more do you need to “follow all the forms”?
And incidentally, I too found Kaylee’s introduction to Serenity to be a bit surprising, but as I thought about it, I wasn’t sure why it should be. Kaylee’s the rural farmgirl type, and, well, we all know that the hayloft isn’t just for storing animal feed.
I also think you have to look at Kaylee’s sexuality as part of a quartet: the four women on Serenity all have very differenct sexualities, and all are presented as equally healthy and equally legitimate–Zoe is the one who likes to have sex, but with men she knows well and is at least fond of, and at this stage is showing us that married, monogamous sex is in no way selling out or settleing. Inara sells sex as a luxury commodity–with no harm to her psyche–and River is a sexless sprite and a complete person without a libido. Kaylee’s sexuality is a sort of indiscriminate lustyness that is no less healthy than the other, contrasting modes.
Going back to the “why would she have sex with that bozo?” issue, I think you guys are missing something: Kaylee has no taste. None whatsoever. My god, did you see the dress she wore in “Shindig”? The goose or the plate she liked on “Safe”? She’s common as dirt: there is no subtlety or sophistication in her. This is what makes her so charming: she’s entirely free of artifice. She sees the good in everything and dismisses the bad as unimportant–practically the definition of “nondiscriminating”. She didn’t fuck the mechanic as part of a plot to see the ship, she fucked that mechanic 'cause he was hot, and while she probably saw at once he was an idiot, she’s not the sort to pass judgement.
Besides, he probably had a tool adequate for the job. 
I totally wasn’t expecting that scene either, but I thought it was pretty funny. After a second or two of thought I didn’t find it out of character for her at all. She’s earthy, in touch with what she wants, has a provincial but certainly not prudish upbringing, and is a very social and caring person. I can see sex for her being like a really warm hug is for some people. She wanted to tumble the guy, so she did. So what if he’s dumb, it’s not like he’s going to be sticking around anyway. Just right for a strictly temporary fling.
Something I didn’t see anyone mention yet is that the mechanic (besides being good looking) was someone new. On a provincial world with low population and population density and probably only a few ships docking at infrequent intervals, meeting an actual stranger would be a rare treat.
Kaylee would have seen him as an opportunity to get news from other planets. We’ve seen how excited she is to see anything on the more settled Core worlds. Depending on her local culture, she might get some social status out of bagging him. He’s a good looking, out of town, exotic guy. He probably could have had his pick of several girls, if the bars by the docks are anything like what I think they are, but Kaylee got him. Look at pretty much any high school anywhere to see that kind of feminine competition.
The fact that he’s crewing a space ship would be almost incidental to most of Kaylee’s peers. I’m sure that was a huge bonus for her though. I got the impression that either he was on his way to his cabin with her and she asked to see the engine on the way, getting hot in the process and dragging him down right there on the deck, or that he and she had been doing it on a regular basis for the time Serenity had been stuck there. The second possiblity would even fit in with Fromage’s initial impression of Kaylee’s character.
I don’t think that she can have been enjoying the engine room with him for that long - otherwise she probably would’ve fixed the engine before they got caught by Mal. But you’re right, the mechanic is a visitor from a different planet, and therefore Kaylee would be interested in knowing more about him (and the world outside her particular rock). I’m not sure if it’s ever touched on - would travelling on Serenity have been Kaylee’s first trip off-world? If she isn’t very well travelled at the time, the opportunity of going on such an interesting ship would’ve been very tempting for someone so inquisitive.
Manda JO, I think it’s a bit harsh to say Kaylee has no taste whatsoever, although I take your point - she’s not too bothered about this guy being an idiot, she is interested in his good points (of which he has many). The dress from Shindig, for example, wasn’t something tucked at the back of the shop that the rest of the crew thought was horrible - they admired it with her (well, the girls did). The goose was terrible though.
I don’t think they admired it with her, so much as humored her. That dress was heinous.
Well, she made it look good.
Was the mechanic an off-worlder, though? I had understood that that was the same planet where Mal bought Serenity in the first place, and that he needed a hotshot mechanic to get it off the ground at all. That would mean that Kaylee, Serenity, and the stupid git were all from the same planet.
And Kaylee learned engineering from her dad, so it’s quite possible that she’d been offworld with him before. But she’d probably not travelled all that far, and certainly a touch of wanderlust was part of her thinking. It wouldn’t need to be all that big a part, though: Engines do, after all, make her hot, and when Mal found her, he pretty much offered her her dream job, even if she hadn’t wanted a bit of adventure.
In this specific case, I think its actually a Willow-Faith problem. Seriously, there was a thread a while back where someone had a problem with the same scene, because that’s not how Willow would have acted, and Kaylee was “supposed” to be the show’s Willow.
You forgot about the women:
Zoe: Taken
River: Too young and crazy, to boot
Inara: …
I’ll be in mah bunk…
I agree with Manda Jo - she wasn’t trying to “get” anything out of the mechanic other than a good lay. She doesn’t use sex to get what she wants - sex is what she wants and bad mechanic guy was really hot. She wasn’t using him to see the ship, that kind of thinking is distinctly not Kaylee’s.
The scene didn’t bother me at all. It seemed very much in character for her.
Every single guy to whom I’ve showed the pilot episode has asked me to back up and show again the long moment where Kaylee takes the luxurious bite out of that berry.
In slow motion.
Using DVD Zoom.
I always, always oblige.
I always felt that, when in the space station freak show with the alien cow ,Simon went into his spiel about her being the only (available) woman in his universe he was as much talking about his being aware that he was pretty much the only (available) man in her universe (Jayne not withstanding). Granted, presented in his usually inept way.
Did anybody pick up that Jayne, at least in the first few episodes, seemed to be very protective and jealous of Kaylee? I got the distinct impression that Jayne had a bit of a crush on her (as opposed to Inara, whom he merely lusted after, hence the famous “I’ll be in my bunk” line).
Maybe Kaylee and Jayne had a fling when they first met. Jayne definitly would’ve made a pass at her, and while a lot of us assume that Kaylee would’ve had the good taste to refuse him, it’s perfectly in character that she didn’t. Maybe this is yet another little fragment of backstory that never made it to screen.