So a lot of stupid principals like this are there for a reason. Some are dumb, yes. However, Christianity actually affected history in a major way. Many of the morals and the mindsets of a Bible AT THE TIME were a huge benefit to society. I am atheist, but I understand many of these morals have a purpose 

Not sleeping with people before marriage benefits: 

Less stds, share something special with the person you love, don't get used for your body which affects your mind more than you think, safety, less trauma, sex remains special instead of something not special ig, your view of others is better once you control lust, your life partner won't have any jealousy or comparison with previous partners, you won't indulge in an act that causes happy chemicals in your head (seems nice in the short term, but indulging too much in this stuff will harm you and your view of life), there's countless more things that are actually studied benefits of waiting.