How do you describe graphic scenes on your writing?

That is a catch 22 situation. It is very hard to draw the line when speaking about sensitive issues. But i would suggest you rather talk about the mood in the room or situation, and avoid talking about the intimacy or the specific experience that the characters in question could be experiencing. 

It depends on what type of story and who is the audience, which tells you how graphic you might suggest your words, as you want the reader to experience the story.

Outlining the picture or the background, the feelings suggesting what took place, also works.

It is more about how you set the tone and mood of the scene. Depending on the target audience, you can go into as much detail as necessary but there are other ways to inform the readers. To spice things up, use indirect cues. It may help you in avoiding some details.

When you are watching a horror scene, the graphics alone doesn't scare you. The music, suspense, etc., everything counts.