I really think that writing sex/intimacy scenes needs to start in a progression. Trying to go from nothing to full disclosure is very difficult to do. Start with sweet stories – hand holding, not much more than kissing – and build up your confidence and experience.
To be honest, I would recommend checking out the book “How to Write a Dirty Story” by Suzie Bright and other books about writing erotica. They have lots of information about how to write sex scenes.
I write of a lot of erotic scenes for the online role playing games I’m involved in. At first it was difficult to do, but I find the more I read scenes in other fiction, the easier it got. Plus, I run the scenes through my head a lot, picturing it as if it was a movie. I find that helps a lot.