Sorry, but I doubt you'll find anyone here willing to write for you. People just write for themselves or their audience. Unless, of course, you're willing to pay them.
And, as said above, if you want to write sex, then just do it. It's not super necessary to start out by adding to someone else's story - what's more important is writing about something you find interesting. But if you do, you'll find you receive a lot more feedback and attention than you would if you wrote on your own. Public stories are significantly more popular than private stories.