The reason why (most) people have sex for non-reproductive reasons is because of the enormous connection it creates between partners. (I say most because there are some people out there who don't have healthy sexual practices.) If you truly love someone, there is no greater feeling than having sex with them, it's a pretty universal human experience. So why do people insist that it's sinful? Is it sinful to close your eyes on a cool fall evening, drink in all your senses have to offer you, and feel truly happy with yourself? I would expect most people to say no. So why does a sensually stimulating activity that often results in true feelings of happiness generate so much conflict? That's the secular question. For the religious, I'll take this a step further - Why would God want to deprive us of this happiness if he loves us?