It's the equivalent of erotic writer to the question: "How do you get your adult ideas?"
My answer? "I have no idea!"
I am an intuitive writer. I sit and write. I'm not sure how I do what I do, and when someone asks me to try to break it, I'm often at a loss. For me it's like telling someone how to breathe. Just ... do.
But if I have to try to explain as it is mine to get from point A to point B (I'm scary in the ninth grade "seems to be working with" flashback math now) I can do. The end. So this is how. It may not be how you do it, or how someone else should do it. The way I do, and perhaps will help you or someone else there too.
THEY ARE ALIVE!
The characters are alive and not the sum of its parts. Are no measures or hair color or size of the penis? I've done sex scenes without mentioning any of the above. Do not ask, "What does my character in this situation?" Allow them to act. Let them decide. Let them talk. Leave them alone. Above all let them know.
GET TURNED ON
If you're bored writing a sex scene, your readers will be bored. If you're turned on, your reader will be turned on. The emotion you are feeling will be conveyed on paper. It's a natural law of the writer universe. (This applies to any scene, not just sex ones, by the way. If it moves you to tears, it will move the reader as well).
DO NOT STOP
If you are turned on dating during a sex scene, really get into it, his fingers fly over the keyboard, unless the house is on fire, or we are under nuclear attack, Do not Stop. Never quit amid a sex scene. (This rule applies even close to actual sex). You'll lose your momentum and it will not be the same when you return to him. Your mood will be moved, and the reader feels it.