If you're writing a sex comedy, and you need a "funny" sex scene for shock value, I want you to go right now and check out this Wikipedia list of paraphilias.
As the article explains, paraphilia is "defined as powerful and persistent sexual interest other than in copulatory or precopulatory behavior." In plain English, it's a catagory that means "weird sexual fetishes."
Here are some of the more interesting ones on that list, with the titles, followed by the unusual cause of the sexual arousal. I invite writers to pick any one of these:
Abasiophilia - People with impaired mobility
Acrotomophilia - People with amputations
Agalmatophilia - Statues, mannequins and immobility
Autoerotic asphixiation - Self-induced asphyxiation, sometimes to the point of near unconsciousness
Chremastistophilia - Being robbed or held up
Dendrophilia - Trees(!)
Emetophilia - Vomit
Formicophilia - Being crawled on by insects
Ursusagalmatophilia - Teddy bears
As you can see, there's quite a list to choose from. Unfortunately, most writers don't give it that much thought. Screenwriters often think they're being edgy and clever when in actuality, they're merely reusing an idea that has been done to death. Case in point, whenever a sex comedy calls for an "oddball" sexual deviance, there's a 99.9% chance that a writer is going to pull out a plush suit and subject me to yet another furry joke.
What's a furry, you ask? Well, in short, it's a sexual fetish that involves dressing up in an animal costume while having sex. In other words, it's people who like having sex while dressed as Yogi Bear and Elmo from Sesame Street.
(Dear god, I really don't want to see the Google searches that this post is going to bring to my page!)
Bizarre? Yes. Funny? Maybe when it's done for shock value. The fact is, this joke has been told many times before, not just in spec scripts, but in actual shows. It's been done on ER, CSI and Entourage among others. In other words, it's gonna take more to get me to laugh than showing Johnny Drama taking a girl from behind while wearing a pink plush bear outfit.
So strike out on your own and pick one of the more obscure "freak" fetishes. Or invent your own. If there is actual documentation on a guy who likes to have sex with trees, then surely your own imagination couldn't come up with something less plausible than that. Be creative and show me something I've never seen before.