Another writer gave me an exercise that helped tremendously.
You start with one man and one woman (it’s easiest to learn the basics first, change it up later). It’s up to you whether they have a previous relationship or know each other or not. Starting from the first time one looks at the other, and progressing to the beginning of their first hot kiss, write four pages (double space, 12pt TNR, 1inch margins), from one POV. Include all five senses, plus mental and emotional input, too. Next, four more pages from the first hot kiss and including only above the waist touching. Same as before, all 5 senses, plus emotional and mental. Next, another four pages, everything up to penetration. Then four more pages with actual intercourse, orgasm, and end at afterglow phase. Smooth it all into one cohesive piece. Next, repeat all the above from the other character’s POV. Repeat as many times as it takes to be comfortable writing sex, varying circumstances, positions, etc.
It takes some getting used to, using so much detail, but when you add in all the senses and thoughts and feelings, those four pages are really short. Of course not every sex scene should be that long or detailed, but once you’re good at writing it all, you can work on paring it down to the essentials.
Also, I don’t have the link on this computer, but Google Stacia Kane’s “Be A Sex Writing Strumpet” blog series. She also compiled it into an Ebook. This series is amazing, tells you how to write an explicit scene, as well as one that’s incredibly hot without being explicit.
AbsoluteWrite.com has a forum for Erotica, including a password protected Share Your Work area, where you can post your work and get great feedback from a group of fantastic writers, many of them published and selling very well.