Maybe because it's a personal and private topic and some people like to keep it that way.

Er... well... ah.... um....

It is because talking about sex is socially ambiguous. It is hard to talk about sex with someone without thinking about them in a sexual way. It can easily be the opening move in an attempt at seduction (or even rape). You may just be curious about the topic, but the other person does not know. If someone you didn't know well suddenly asked you "How much money do you have in your wallet?" what would you think? It might be someone who was just curious to know how much money people carry with them. But it might be someone who was trying to decide whether to rob you. If you were not sure, you would hesitate to answer.

Maybe there is based on different culture bakground that affected peoples feeling and ideas then they will avoid to discuss with it , like China , preaent in China they are weak in sex education with their children when they were young .

Not only humans but also other primates usually don't talk about sex.