In any story or movie, there is always a climax, which no pun intended means the highlight of the story. In a sex story the climax means the climax! The height of orgasm, the whole reason for the story in the first place.
You need to be aware of this as the beginning of the story continuously builds up to the point of orgasm. Once the orgasm finally comes, you need to make it big and important otherwise it will be anticlimactic and you will leave your readers with blue balls they will be wanting more and will not eel satisfied.
Make sure you enhance the orgasm and make it better then real, since this is a story, you can do whatever you want. Be very descriptive and talk about screaming, rolling the eyes back, clenching of feet, squirting, scratching or whatever else you can think of that denotes a very powerful orgasm.
Both characters need to be addresses, the man will have an orgasm, too so you need to mention how he feels. Keeping in mind the smutty level of the story, you can describe in vivid detail or somewhat low key about the events.
Make the orgasms big and proud, it's the highlight of your story!