This isn't set completely in stone, but is a pretty good baseline for what I'll go for. Let me know if you've got any questions!
Yes--
*a little fumbling and awkwardness-- either verbal or physical... come on, it can't all be perfect!
*some CR initially (at least exchange names first!); I think it's fun to see how characters interact outside the bedroom sometimes, too! I'm fine with writing out just plain CR threads, too, without smut.
*lots of foreplay. I'm one of those weirdos who enjoys writing out the foreplay much more than writing out the actual sex.
*... however, that being said, I'm not unwilling to write out the actual sex-- usually I will try pretty hard to follow the other person's lead in terms of how descriptive they want it to get, but I don't really like the old one-sentence response.
*cute romantic sex/cuddling
*... also somewhat rough sex
*fingering
*oral sex (whether eating out or a blow job-- funnily enough, I tend to write a lot of oral sex scenes for whatever reason)
*mild to moderate acne dubcon
*light, improvised bondage
*walling
*doubles
*yaoi, het, and (if sexswitched) yuri is all good (i.e., anal, vaginal, etc.)
*actually I confess to a disproportionate fondness for sexswitch (regardless of whether the smut produced is technically het or yuri)
*as far as sub/dom stuff goes-- Katou tends to usually be more dominant with girls than he is with guys (or would if he ever slept with any guys that weren't stuck in girls' bodies-- 'sup, Setsuna), but either way he's something of a flailing mess no matter how long he's been stuck in a love hotel. =(
*noncon
No--
*really, really strange read: huge toys (dildos, vibrators, strap-ons are generally fine, but I'd really rather you ask me first regardless)
*lactation
*tentacle rape
*fuuta
*furries
*characters under the age of fifteen for smut (Katou's only seventeen, for the record, but the fifteen and up rule still stands)
*scat/golden shower/vomit/etc.
*bloodplay, at least under most circumstances (mostly because this guy gets cut up enough in canon without me adding to it)
*penetrating non-sexual orifices (i.e., skullfucking)
*vore
*infantilization
*rimming
*erotic asphyxiation
*piercings
*snuff
Weird Technicalities--
*Poor Katou is not human. He's the soul of a dead human transplanted into a plant/mechanical hybrid body with a side helping of angel. But his body functions close enough to a normal person's, and for the record he gets dubious compliments on how good his construction is. ='( For the record, technically he's an angel, so if your character can sense stuff-- that's what they'll sense. Katou's own ability to sense auras is kind of dubious; he can generally tell where Setsuna is but isn't seen doing much more than that.
*Katou... is not a picky person and most of his preferences rest on how your character treats him rather than any specific physical qualities your character has (maybe a vague penchant for pretty people but-- that's kinda universal). If you're nice to him or even mean-with-an-occasional-side-of-nice, chances are he'll like you a whole lot more than he'll ever admit, although not necessarily with a romantic bent. Having had a hard time of it, he's not used to being treated too well and... ugh, just read the manga. It's seriously pathetic how little it takes to wrap him around your finger forever. ... Of course, you don't have to treat him nicely. XD He's also insanely obsessive. But luckily it's only over one person.
*Katou has two main abilities in canon which are vaguely relevant to TLH junk:
1--flight (three white wings usually hidden behind his shoulderblades-- these things are 1. gigantic and 2. apparently erogenous zones)
2--shapeshifting into other people he's seen (which is much more relevant to the canon plotline), requires concentrating on the form. According to one of Kaori Yuki's author's notes
here she actually thought about having him end up shapeshifting into a girl at one point, so yeah, that's enough indication for me to think he can manage genderbending.
*IF YOU REALLY WOULD LIKE TO, let me know somehow and either ability will be glad to rear its head, so to speak, but... don't blame him if he's mad that you only want to have sex with him when he's shapeshifted into someone else. Or turns you down. The wings you can probably coax out of him a bit easier, though. =)
Caveat to the shapeshifting-- Katou has very severe memory problems (they are canon) and might have difficulty concentrating on remembering the form at first. Alternately his memory loss might have (and has during one point in canon) gotten so bad he'll forget who he is/what he looks like and instead assume the form of the first person he sees until he recovers his memory. Feel free to help him with that if it ever comes up. ... Or just convince him that he's actually a pretty, pretty princess. Whichever. XD