With all the good advice above I would like to add that it is vital that the sex be true to the motives of the character. How the character responds during the act should tell the reader that much more about them. Take for example a boy finally getting to be with the girl of his dreams, would he describe the first time in clinical by the book fashion? Probably not, if he did there better be a damn good reason behind it, a deeper character motive. Similarly lack of reaction is very telling as well. As stated, not all sex is good and some sex is just downright bad.
You do not have to have had sex to understand some of the feelings and emotions involved. There is excitement, fear, longing, frustration, pain, anger all of it can happen during sex and showing these can go much more toward moving your story, and your character forward than any well described sex act.