Many 'Laws simply reflect an outdated morality than any actual protection for the individual.
A prime example is prostitution- why exactly should the State interfere in sexual relations between consenting adults where no coercion is in force?
The other reply mentions 'social concern'- a nebulous concept since it is individuals who are harmed by enforcement of these type of laws, and the wider public should have no concern over the dealings of individuals anyway.
Morality is usually bigotry with a veneer of righteousness.
No, do you think the government has set out the rules and regulations in regard to crimes, for fun? They're there for a reason, the law on precedent has been so well developed that if there ever was a scenario in which a court felt a certain act SHOULD be decriminalised, they can set the law via that case and judge the next similar case accordingly. Some people may argue that some laws are too harsh or too lenient - but at the moment the government have found almost the perfect balance between social concern and themselves. So going against over 200 years of law making would jeopardise that bond and thats why parliament do not take that risk.