Why there are so many"techniques" about sex in Taoism?

Unlike any other Chinese ancient philosophy, Taoism teaches many techniques about sexual practice. By my understanding, this is because Taoism focuses mostly on human's physical health. Ok, don't get me wrong, it is not that Taoists don't care about spiritual matter, but they believe that physical health is the fundation of our mental health.
Well, did Lao zhi really said that? I am not sure... but this is how I understand Tao.
By the way, I am personally not so crazy about this part of Taoism. I think if you exercise often and eat well, you will have awesome sex!

Question about China: #1 Are Chinese people believers?

I think it depends on the definition of "believer". If "believer" means who believes only One God (monotheism), or believe lots of Gods (polytheism), I would say NO. There have never been such kind of religions existed in China. Or if there were some, they must be too weak to be influential. However, if the meaning of "religion" or "believer" in your dictionary is border than those I listed above (monotheism and polytheism), you may consider some of Chinese are "religious" because they believe or even practise Buddhism and Taoism. The former is closer to religion than later, by my opinion, due to its' more strict on its practices.
Even though there have not been any "serious" religions in Chinese history, there were some ancient myths about the creation of universe (Nüwa http://en.wikipedia.org/wiki/N%C3%BCwa).