My female characters have periods, my male ones can't always get an erection to order, not everyone exists in a permanently aroused mood, and not everyone who meets has sex together. Have said all that, I never allow too much realism to get in the way of the story. It just wouldn't be possible! I have done most of it though. Anything to do with straightforward sex, pony-girls, corporal punishment, exhibitionism or panty-wetting I'm describing by putting real experience into a different context. There are some things I'm just not into but enjoy describing, like piercing and tattoos.
When I want to describe those, I ring a few friends to get a consensus of opinion, but more often than not I find that everyone's experience is very individual. Other things are better in fantasy than reality, like anal sex with a string of partners. I don't do anything that risks infection, especially with HIV. I also take a bit of licence when it comes to describing heavy scenes, although I try to keep at least some grip on reality. I know I couldn't handle being buggered by three men in a row and then strung up my wrists for a whipping, but it's nice to fantasise about it. In fantasy anything goes, in reality I stick to safe sex.
When the terrifying "Miss Marchmont" asked me the same question I decided on an occasion when a girlfriend unexpectedly gave me an enema while I was masturbating in the bath for her. On the other hand, I didn't get much say in what happened, so I'm not sure it counts. I'm not a swinger, so I've never sucked off ten men in a row or anything like that. It's always kinky. When I was seventeen, and a virgin, a boyfriend pinned me down and tossed off in my face, which was such a shock, and certainly filthy, but again, not my own choice. I've been spanked in public, which is an incredible hit, but not really filthy. Actually, for sheer, inexcusable filth, doing my toilet in my panties and then masturbating through them has to take the prize. Just thinking about it makes me blush.