It will come as no surprise to most Americans today that most people engage in sex before they are married - and this is true of members of all religions as well as for agnostics and humanists. However simply because a behavior has become commonplace does not mean that it is acceptable, or at least that it is acceptable for all people. Christians who wish to be true to the teachings of their church should not engage in sex before marriage regardless of what the cultural norms may be for other groups.
The reasons that Christians should not engage in premarital sex are numerous. The first of these is simply a question of morality. The Bible provides a number of instances in which individuals are either implicitly or explicitly told not to engage in sex with people with whom they are not married: We are enjoined by the Bible to treat our body as if it were a temple, given to us by God and made in the reflection of a divine image. By honoring the divinity that is immanent in our own bodies (and this includes a number of activities such as not taking drugs) we honor the original act of creation.
This is true of all Christian sects, although more so in some than in others. It may be obvious but it is still worth mentioning here while are addressing the issue of premarital sex from a Christian perspective there is of course no single Christian perspective: Unitarians have very different beliefs than do Catholics, for example. However, all Christian groups argue that it is immoral to have sex indiscriminately because to act as if our bodies were not gifts from God is to dishonor the entire concept of original divine creation. Seventh Day Adventists are perhaps the most strict of all Christians in this arena: They argue that not only should Christians avoid sex before marriage but should be in all ways reverential to their bodies as a way of honoring God, making a connection between a prohibition against premarital sex (widespread among C...