Social norms vary between cultures and have throughout history. In most societies, the expression of sexuality is demonised to a certain extent. This is apparent in the shame brought to people engaging in premarital sex within for example a strictly religious family. There would be instances where sex is not regarded as something potentially dirty, but it certainly seems that, historically and across cultures is has a propensity to be a taboo subject.
Sex is a fact of life, we have genitals, libidos, and sexual intercourse is essential for continuation of the species. It is an emotionally charged subject, subject to wide and varied moral judgement. The underlying cause of many psychological problems (fetishes as a more obvious example), sexual jealousy can underlie much violent crime, sexual deviance can destroy and wreak utter devastation in lives (for example pedohpilia). The are issues of repressed sexuality a thriving pornography industry, unwanted pregnancies, sexually transmitted diseases and yet the open discussion of sex and sexuality can still be regarded as a taboo subject.
Why is it that people have this tendency to treat sex as a dirty or taboo topic and one that cannot be openly discussed?
The question is connected to this Is there a causal connection between repressive sexual morality and antisocial behavior? in asking the follow up question:
Is there a correlation with a person's uncomfortability and objections about open discussion about sex and unresolved sexual issues?