Writing Sex : Passion ...? or Purple Prose?

So I have finally reached the point in the manuscript of my novel-in-progress where the heroine and her about-to-be-lover are going to consummate their star-crossed romance. It's 300 pages into the narrative (much else is going on, as it is based on historical events) and as I search for all the right words I also began to second-guess myself. Have the characters earned this? Have I shown them earning it in their previous scenes? And what about the words themselves? Am I being overly lyrical? Too overtly graphic for a historical setting? Are my sentences reading mawkish or maudlin? Am I feeling what my characters are feeling?

Some of the hoydens are masters (mistresses?) of the powerfully erotic sex scene (I'm talking to you, Pam and Isobel). Do you (and the other hoydens, and our visiting authors) ever struggle with the words, or do you find writing sex scenes one of the easier aspects of crafting your novels?

From a craft standpoint, do you have any rules you apply to your scenes (e.g., if they make you giggle when you read them aloud, you start rewriting?) Do you tend to go for the graphic, or employ euphemisms? Given the fact that there are a limited number of body parts and names for them, and unless your characters are well versed in the Kama Sutra, a fairly narrow repertoire of positions they'd be familiar with, how do you keep these scenes fresh, even when they are character-driven?

And to both authors and readers ... what makes a sex scene in a novel work for you?