It's a little older - well, like 20 years old now - but I read Susie Bright's How to Write a Dirty Story and found it a really helpful guide to getting the sex on paper. It's not easy to find but it's the first resource I thought of.
This article contains some stuff I don't agree with (or care about), but the "Show Don't Tell" section is a good summary about the mechanics of writing a sex scene. I think that showing not telling is the biggest hurdle to good smut writing, when it comes to the squishy, sloppy details.