I have an idea for a new story that I want to write, but I'm having trouble getting around the particulars of how to present the story. It's about a man who stumbles upon two people in an alley having sex, written from the point of view of the man who is spying on them. I'm debating having the story revolve around the masturbatory side of things, accentuating the pleasure he experiences of watching two strangers have sex, though I'm wondering how exactly to convey that. Do I focus on the pleasure of him masturbating, fantasizing about joining them, or should I focus more on the sexual escapades of the two people he is watching? I want to make it as realistic as possible while still staying confined to the first-person perspective.
I've written several rough drafts utilizing different approaches to the scene, but not one of them captured me enough to continue writing the scene the way it was going. Does anyone have any advice they could offer?