Its not as simple as there being just two groups of people with ideas that are opposite each other. Each party incorporates a range of political beliefs so that they can have as many members as possible. They win elections, after all, by getting the most votes.
So the Republican party has libertarians and right-wing fundamentalists (who are on the opposite end of the social spectrum), while the Democratic party has people who believe in complete government non-interference in social affairs and people who want more government interference in things like healthcare.
This diagram explains it better:http://weblog.leidenuniv.nl/media/blogs/76039/invisiblecollege/political_compass.png
I disagree that the UK parties, and politiciansi in general do not get involved in social affairs. They just seem to be more in agreement about them than the US parties are. We would not have things like Civil Unions if people in politics didn't vote to make them legal. And wasn't there a discussion about restoring tax breaks for married couples? Taxation is often used as a means of social control. Laws prohibiting the use or sale of drugs are also examples of social control.
In fact, it might be better for politicians to constantly be arguing about what is right and wrong than for everyone to agree on what is right, so nobody has to think about it, or even realize that by not thinking about it they have made a decision by default. If you constantly have to justify your beliefs, you are going to have to adapt them to fit changing situations and new information.