Having sex before Marriage is not just a sin,,its even against the Pure Human nature,,thats why all the mankind who did this without marriage or before marriage they all (even those who admit and those who hide this) but they all naturally feel the Guilt and know very well how they did something wrong..this because God created us by the pure and clean human nature which is against sins and especially the sexual ones,,as its more sensitive act..and Females feel this kinda more than males,,but they all feel the same thing by one way or anther inside themselves..and Yes its a sin according to God and all his prophets teaching { including Jesus of-course (pbuh) }..and i think,,,for SURE you know this already before asking..(no offense)..but kindly,,try to learn about this new fact,,as you will NEVER FIND any verse at the Whole Bible or in any of Jesus words,,where God or Jesus and any Prophet asked from anyone to be a Jewish or a Christian,,you will never find this,,not a single verse ! but you will find how all the holy texts and all the prophets including Jesus also,,,they all taught their people to be { a believers whop submit to God (Te Creator) and his will }..and its called in Arabic (Islam) ,this is the First and only true religion,,but unfortunately the Christianity which we see now is man-made and even against the Bible teaching..