Why is it Biblically wrong to have premarital ?

I'm a little confused as to why this is a sin. I don't agree with being promiscuous or unsafe, as this leads to diseases and unplanned pregnancies, but it's a little unclear to me as to why it is a sin to engage in premarital with an exclusive, long-term partner. From what I understand, when the Bible was written, people were married in their early teens and were married to someone they hardly knew (I could be wrong). In this case, it is not such a struggle to abstain from until marriage. But these days it would be considered stupid and naive to marry so young, and most would say it's a good idea to get to know the person for a few years before making the decision to marry. So is this a sin just because the Bible says it's a sin, or is there some logical reasoning to back this commandment up?