Sex is a simple word for a very complex set of desires. It cuts to the core of our passions, our wants, our emotions. But when it goes wrong, it can be the most painful thing of all. Professor Alice Roberts looks through 45 years of Horizon archive to see how science came to understand sex, strived to solve our problems with it and even helped us to do it better. Can science save the day when sex goes wrong? (BBC FOUR)