Write a sample playthrough of your sex scene. List the commands that a player will do and write the responses that will happen. Think about alternate commands that the player might try. Here is a sample transcript:
> kiss her
You kiss her.
> Lick breasts
You gently lick her nipple.
> Have sex
You have sex with her. You cuddle with her afterwards and go to sleep.
It shows different commands that you can try and the expected responses.
Writing sex scenes can be difficult. Surprisingly, writing sex scenes isn't taught in school, so most people don't have any practice with it. A good sex scene shouldn't just be a mechanical description of sex acts. Describe how it feels and the emotions involved. I am including numerous excerpts of some public domain sex scenes that you can pilfer and adapt for your own writing.