Because, Tia, politics is a multi-axis matter. The division between left and right as it applies to politics generally refers to the degree of economic control by government over the people. What you're talking about is the morality or private behavior axis, i.e., how much control (and in what direction) government has over the personal activities of the people. Also, you're more familiar with the American divisions into what's loosely called conservative and liberal, but more accurately should perhaps be called traditionalist and utopian. In America, both traditionalists and utopians want collectivist control over individual freedoms. Libertarians prefer fewer controls on both axes, and generally favor individual freedom both in moral (personal activity) and economic areas.

So in reality, both conservatives and social liberals have collectivist leanings in those areas. Many conservatives are moral traditionalists who would like people to conform to a certain version of morality in sexual matters. Many liberals are social utopians who would like people to conform to social utopian limits on speech. Luckily (I think), in America the First Amendment puts a check on both of them so we don't, as Canada does for example, have laws that criminalize the use of certain words or the expression of politically incorrect ideas, and we no longer criminalize various types of consensual sexual activity between adults. Accordingly, in America the big battle right now is in the area of economic control, which is not restrained by the First Amendment.

If you want to talk about the moral/personal activity axis of collectivist control over individuals, you should start a different thread because it's really a different issue entirely, although once again, briefly answering your original question, it does reflect that basic conflict between wanting freedom for oneself and wanting to control others.