Sex is either extremely easy or extremely hard to write. I still have not figured out which. To some people, it comes naturally. I mean, there are only so many positions you can make love in, after all, and the general motions are usually the same. Writing them out isn't that difficult. On the other hand, in order to write a good sex scene, you have to be in the right mood, you have to have the right characters, the right location, the right words... you have to be inspired. Putting all that stuff together is hard work. Not to mention that we all feel embarrassment at some point when we imagine our mother reading what gets us going.
Here are the most common questions I get about sex scenes... *drum roll*
Qusetion: What word should I use for the female anatomy?
Short Answer: None.
Long Answer: It ALL depends on the scene and the characters. I can only tell you one way that I, as a reader, view certain words and their effect on me. Vagina completely turns me off. It reminds me of a trip to the gynecologist. Never, ever use this word or I will hit you. Pussy doesn't do it for me, either. Too cutesy. Try to use this word rarely or not at all. Cunt only turns my crank if used EXTREMELY sparingly, and only in BDSM scenes where dirty talk/control is one of the main elements. Once in a blue moon, I've seen it used well, but besides that... avoid that word, too.
Try describing specific parts of the landscape instead. It's a way to be more erotic! "Thrusting inside of the blonde/redhead/brunette" is better than "fucked her pussy/cunt/vagina". You might find that using a clever turn of phrase to AVOID a specific word will make your writing more erotic. Sorry if you are writing about a dude, but I have no suggestions for other words for 'penis'. They all make me giggle. I find penises highly amusing, but not erotic at all.
Question: When should I include my first sex scene?
Short Answer: Around the middle of the book.
Long Answer: Whenever you think it's right. Some romance novels start off with a sex scene. If you are writing a sequel, this will not seem strange at all (think Radclyffe and her webs of couples... the main couple usually bangs in the first chapter. The sexual tension is taken care of with whatever new couple she is hooking up, since she writes about multiple couples at once). Other romance novels don't add the sex scene until the very end. This can work, but building up to it is difficult, and you have less room to play. I advise going for it around the middle or 2/3rds of the way through. It just feels like a good spot to me.
Question: How long should my scene be?
Short Answer: As long as you want it to be
Long Answer: It varies. Factors that might influence your choice are the scene's location or its purpose in the book (yes, even sex scenes need to have some kind of purpose in the book...) Some of my favorite sex scenes are only a few paragraphs long. They have to have punch, though, for that length to work. Or you can go the long-winded route and make your sex scene take up an entire chapter (or two). This can be fun as well. Just go with it, and don't be afraid to experiment. It's fun!
Okay, that's about all I can write on this subject for now. I'll probably add to this topic tomorrow, it's a big one. (No sexual pun intended).