Why Is Sex Before Marriage Not Allowed In Islam?

Well it is not allowed in any religion. People do what every one around them is doing.

The Logic behind not allowing such act, is to maintain a healthy and strong society.
To explain How .. Its a long topic.. You may ask for it

Because you are suppose to stay pure on your wedding night so that your husband can take your chastity. Only way of knowing in women is if their hymen is already ripped before, otherwise you can tell in men.

Simply to uphold the society as a whole. Look at the number of single mothers in the west.... It is an extremely difficult situation when you have to take care of a child(s) and earn a decent living.... Noone would want to be in that situation. When you get married, you announce is so everyone knows that this man and woman are taken. It is much simpler for a man or a woman to sleep around if the relationship is not public. Every child needs to be given a name of the father... How will it happen if the man was not committed and refuge to take responsibility. The whole society crumbles down.... Remember men wont mind as they don't take ownership of the child produced.... Women get stuck. It is for the whole society specially women that sex should only be after marriage.