If you can handle a litty dirrty dirrty on Friday, in media Res, the slash fiction site, has a link on how to write a sex scene. And not a believable sex scene: a sexy sex scene. Before you even click on it, the disclaimer reads "This essay is rated NC-17 (adults only). It includes explicit male/male sex. If this is what you came for, scroll down. If it isn't, hit the Back button." Otherwise, plunge in.