Basically, liberals believe that people should be left alone to do whatever they want in their personal lives. To them, the government should not try to legislate morality. This is why liberals believe that gay marriage should be allowed because there is no (in their view) reason (outside of religious/moral beliefs) to prevent same-sex couples from marrying.
On the other hand, liberals do not want people to be left alone to do whatever they want economically. The believe that the government needs to prevent people's greed from causing them to harm society. Therefore, for example, they want to enforce things like minimum wage laws and they want to require car makers to make cars that get certain gas mileages.