When I refer to the Church, I mean the teachings and beliefs that dominate Christian teachings. There are approximately 2.1 billion people worldwide who identify as Christian. About 1.2 billion of those people are Catholic. The Catholic Church definitely believes premarital sex is a sin. Protestants number 600 to 800 million, and of course, each denomination might teach something different in regards to premarital sex. The majority still teach it's a sin, however. So while I agree that not everyone believes it's a sin (I'm a Christian who thinks it isn't!), enough major Christian traditions teach that premarital sex is a sin, so I'm comfortable generalizing the Church in this situation.