"If you don't like gay marriage, you don't have to have one." So goes a standard admonition in our culture wars. This quip can be applied not only to same sex marriage but also to a host of other controversial issues. The retort assumes that some matters represent personal choices that are no concern of others. According to this view, the larger society should punish criminal activity (murder, theft, and the like) but oughtn't to criminalize other acts, which only a minority believe to be problematic. Gay marriage, then, might violate the religious beliefs of some, but (in the logic of this quip) they have no grounds to prevent others from enjoying its benefits if they wish.
Those who seem bent on applying their own moral code to everyone else view this question differently. At times they even declare that if our society fails to uphold their moral code, it will (or has already) become godless and sinful. The conviction that the neighbor's private life is your business has a deep history. The English men and women who settled New England, widely known by the somewhat inaccurate name of "puritan," believed that each of them held responsibility for the actions of the whole community. This view was so strong that it affected not only religious practice but also the legal system.
Massachusetts preachers told their congregations that God would punish them all if any members of the community sinned. In their view, fire, flood, and other mishaps arose out of divine displeasure. Ministers listed all the sins they identified as possibly causes of the Lord's wrath. The list was very long--and the preachers disagreed often on the details--but sins they noted ranged from a decline in piety to toleration of radical religious groups such as Quakers. Clergy used this idea to encourage their listeners to change, by becoming more pious, less accepting of religious difference, or taking whatever other actions they thought in need of amendment.
Such ideas led to a practice known as "community watchfulness." Everyone was expected to monitor everyone else and to report on what they saw. Unlike the modern "Neighborhood Watch" movement which is framed in terms of the community observing strangers who come to do harm, this practice asked neighbors to spy on each other. Though many of our popular conceptions of the "puritans" are inaccurate, they did in fact mind one another's business in a way most modern Americans would find creepy. Sometimes the spying and the level of suspicion resulted in catastrophe, as when people turned on each other with accusations of witchcraft. After the Salem crisis abated, the community realized that their fears had been groundless, but this realization came only after nineteen executions and the disruption of many other lives. A mandate to keep everyone in line encourages paranoia and distrust.
The residents of early New England gradually dropped this practice and over time they came to value personal privacy. Yet the idea that my sin (as defined by you) is the community's business reaches back to this earlier view that the community as a whole could be punished for the actions of a few. It didn't work well then, but that doesn't seem to deter the advocates of a return to this old policy.