Because religion was used to control the masses. Using fear of going to hell to corral them into doing their bidding.
So among other things, they started to say that sex and nudity are sins and should be hidden.
It promotes healthy relationships, self-esteem and overall happier population.
Yes. We have thousands of years of evidence of what society was like before Christianity rolled up and demanded people act a certain way or they'd be sent to hell. (Including very notable societies like Ancient Greece and Ancient Rome)
There's nothing explicitly wrong with sex. The whole taboo regarding it is simply vestiges of the means of control used in a prior time (After many, many civilisations had been thriving and successful before such a notion came about).
Ye gods. Tell me you you know nothing of religion or ancient history without telling me you know nothing of religion or ancient history.
Your use of the word 'religion' here means Christianity or at least your limited knowledge of the many, many flavours of Christianity.
Thousands of years of pre-Christianity evidence? Do you think that ancient Greece or Rome were some sort of hippy, free-love societies? What about ancient China or India? And once we get beyond about 500 years BCE our knowledge of societies drops off dramatically.
There are/were many reasons why extra-marital sex was frowned upon, from bloodlines to yoga.
