I guess I should start out by saying that what I like to read and what I like to do are two different things. This post is completely about fictional smut, in written form only. I'm restricting the topic to fictional smut because a) I don't have to have done/tried something to find it hot/squicky, and b) I don't talk much about my personal life.
So, let's start with the easy list: squicks. I've already talked a little bit about characters and characterizations that "turn me off", I think, so let's focus on specific *things*, rather than people.
- Bodily fluids other than sexual ones. Actually, blood isn't bad in small amounts, though it certainly isn't *exciting* to read about someone licking someone else's blood and liking the taste of it, unless it's a vampire, in which case that's quite welcome :). And yes, I've read one or two fics featuring golden showers that were incredibly hot... but I would never ask for such a thing. So I guess the real squick here is poop. And vomit, which should never be anywhere near a sex scene, at least not one intended to arouse.
- Bestiality involving non-sentient animals. For the Harry Potter fandom, I don't have a problem at all with intimate contact in Animagus forms, or a Moony under the influence of Wolfsbane Potion. Crookshanks is probably across the line, though. Fang? Norbert? Other pets and owls? Probably squicky.
- Too much attention on girl-bits. I prefer cocks, plzkthx. Also, the word "cum" spelled that way makes me a sad panda.
Not a long list. This is because not a lot of activities squick me. A lot of things are kind of neutral, though, which means if done right they can be hot, but otherwise they fall flat. Incest, for example. It doesn't squick me at all, but neither is there anything particularly exciting about it in and of itself. On the other hand, incest often implies an unequal relationship and something that has to be kept secret, for which see #1 below.
All right, enough of me being wishy-washy on squicks. Let's talk about the hot stuff.
An important distinction: Most of these kinks are perfectly acceptable when applied to a pairing that is not "in love." It can be exciting for two characters to have sex for reasons besides being in love. At the same time, I adore reading fics that involve loving relationships between characters! Anyway, here are some things that are in and of themselves "hot":
- Secrets. The absolute number one hottest line I have ever read in a fic is "And this is so naughty... you mustn't tell anyone about this, Draco... do you understand?" from Daisychain!Draco. Any time you have sex that has to be hushed up, especially if the sex is "wrong" and one person will get in trouble if anyone knows (rather than just because they're very private people, for example)... I for one find it very hot. Can work for couples who love each other, especially when play-acting, but I think it's easier to work it into a fic where there is physical attraction, or power plays or something, but not love.
- Dubious consent. This sometimes gets muddled up with the more general "sex for the wrong reasons." In other words, the way I like dub-con, the person is agreeing to sex, but not because it's really what they want. Forced bonding stories often fit this category, or someone who trades sex for protection or the other person's silence or something like that. I can't think of a really good single example. Anyway, if the purpose of the fic is to arouse, then I have no problem at all with some plot contrivances to get things to work. And AU doesn't turn me off, in case that matters.
- Rape (or "non-consensual" sex). This is obviously a controversial sort of thing. I sometimes find it hot, and I'm still trying to nail down exactly what are the elements that make it hot, as opposed to painful and icky (or dumb and fluffy and stupid).
- Rape fantasy. This is one of the few kinks I have that works much, much better for characters that are in love or in an established relationship. For lack of a good way of explaining this, here's a sort of example. Bill and Fleur are left alone in the Burrow in Half-Blood Prince when the rest of the family goes to Diagon Alley. She turns on the charm, hitches up her skirt enticingly and puts on a sort of display for him. Then when he reaches for her, she submits totally and pushes her arse back against him and murmurs things like "oh, non!" even as she is totally getting it on and being all minx-y. That kind of thing can continue right through the actual sex. Probably would be best if Bill got off on it. I can also read/write this with the man pretending to protest ;-).
- Chan, or adults engaging in sexual activities with minors. I don't usually worry about anyone younger than 11, because 11 is when students go to Hogwarts. I usually prefer for the younger participant to be active and eagerly participating. For an example, see dementordelta's The Devil Will Drag You Under. This kink also ties in to kink #1.
- Cross-generation, which I have only recently realized can be a separate thing from #5, is also nice but not quite as much as actual underage stuff. I think it's because they don't have to keep it secret, although it still might harbor some stigma and therefore might be kept semi-secret.
- Blowjobs, esp. in which the person "receiving" the blowjob is er, aggressive about it? Check out this super-amazingly hot line from curiousqity's fic Open Door:
The fear receding from Lupin's frame, he took more of Severus inside, relaxing against the chair's back, his hips working even as he knelt there with his hands idle, passively allowing Severus to fuck his mouth.
... Yeah. Guh. Or if the man holds his partner's jaw or forces their lips open or something like that... yeah, that's all good. With this I prefer "reluctant" (rather than outright unwilling) - see "dubious consent". Although, I should point out that reluctance is not required at all - this is just as hot with a perfectly eager participant.
I've read plenty of smut that didn't fit any of the "kinks" I discussed and still were fantastically wonderfully hot. So any hypothetical author who was looking at this wondering what to write for me should not take it as a grocery list of "must haves", not by any means.