yes ,sex before marriage is a sin but if we wasn't Christians then we wouldn't know this.that is why we have to have god in our lives so that we can be foregiven of our sins.just like jesus said when people gathered around and was getting ready tons stone a whore.who so ever that hasn't sinned cast the first stone.its up to the couple if they want sex before marriage.and just remember who put us on this earth.and what put sin in our lives.god loves us all that is why he gave his only begotten son to die for us.so that we could have ever lasting life.