[NSFW] How do I turn a self-indulgent fantasy into a good story?

The only time I ever get inspired to write is when I'm in porn threads that turn into general romantic fantasy discussion threads. I finally have an idea I can remember for more than 24 hours and want to make something worthwhile of it.

Am I starting from the wrong jumping off point?

Edit: i just started on my first draft; thanks to everyone that has shared their advice.