Not that I think that without the concept of sin and shame and so on, we'd all be doing it in trees. It's just the fear of trees, isn't it? The fear that if Godly and authoritative types don't say "if you have sex in trees you'll go to hell", that's what we'll all be doing.
But really we wouldn't. Because, well...having sex in a tree is probably pretty uncomfortable. Branches sticking up your bum, bark scratching at the backs of your thighs...that sort of thing. Plus, we're not monkeys.
So poo on sin and shame and all that stuff.
That being said, in the ludicrous panoply of worlds inside my weird head, I do enjoy the idea of sex as a sin. In the modern world, there's not many of those old barriers around sex. Of course some still remain, but there are definitely not quite so many old dudes and pinch-faced ladies, telling us what we can and can't do, sex-wise (thank God, ha ha!). Which gives my head a kind of freedom, oddly, to think up those barriers for itself.
Because those nothing so fun as building up the walls, only to smash them down again.
For example, in my time as a writer I have put up the following sex obstacles for my characters:
1. Magical powers mean that if character has sex, other person will die.
2. Magical powers mean that if character has sex, he will die.
3. Magical powers mean that if character has sex with another character, both of them will die.
4. Character does not know what sex is due to horrible apocalyptic happening, and is afraid of it.
5. Character does not know how to have sex with a woman, because women ceased to exist due to horrible apocalyptic happening. (This one has been published: it's out on Monday from TEB. )
6. Character cannot have sex with other character because other character is an alien from the planet Blargon.
Of course, I've hugely oversimplified the plots of my stories (I hope!), but at their core, they do represent my fascination with things that stop sex happening. And though this fascination isn't all about sin, sin does make a heady stopper, just waiting to be overcome.
All I have to do is look at this picture:
To know that.