Like every creative craft, there's always an urge when writing erotica to go one better than the last scene you wrote, and it tends to result in some very challenging bedroom segments that you spend hours trying to get just right.
When I first started writing sex it was purely a case of splurging out a ton of stuff that I found totally hot all over the page -- and that isn't necessarily a bad thing when you're writing one-off smutty shorts that're aimed at cheap and cheerful thrills. With erotic romance, however, there's so much more to the anatomy of a sex scene than meets the eye.
When writing vanilla segments of a story I can usually put out a good two thousand words a day without much slowdown, but when it comes to the sex that output tends to get cut in half. The story is the bread and butter of an erotic romance, but the sex is the succulent filling. It's the focal point, where you can be sure you'll have your reader's undivided attention, and where a good author will be able to create a memorable sequence that will stick with you long after you finish the final page.
As a result there's an almost painstaking urge to pay that much more attention to writing a sex scene than to any other part of the story. Every word choice has to be sexy, the pacing has to be spot-on, the emotions of the characters involved need to be crystallised and poignant. The bedroom scenes I've written in the past that I've been least happy with (in eRoms, at least) are the ones where the sex feels forced, inorganic, or irrelevant to anything beyond the scene at hand.
Needless to say, it often feels like a pretty daunting task to get the sex scene in a chapter of a serial right. It's a more directly climactic (hurr hurr) moment than almost anything else in your writing, and once your mind starts to be bombarded by all the things you feel obliged to include in it, the simple process of sitting down for an hour or two and bashing out a sex scene starts to become a lot more complicated.
I'm at the point now where my sex scenes almost always take multiple writing sessions to get finished, because they tax me a lot more than anything else. That's not to say they're tiring or exhaustive to write (although feeling a little breathless by the end is never a bad thing!), but a good sex scene is definitely a challenge to get right.