The simple answer is that the Qur'an, which we believe comes from God says so. But that doesn't satisfy your inquisition.
Some prohibitions in Islam are not explained as to why something is prohibited. It becomes apparent once they are committed. The visible results that arise from this act is an increase in STD's, destruction of families and the lives of children having to grow up in torn up families, alack of love between two people, etc.
Although married couple do get divorced and their children might go through the same hardships and difficulties, the rate is and can be drastically reduced by prohibiting fornication. Islam is often criticized for it's allowance of marriage for both genders once they reach the age of puberty (taking into account some other factors too) but the fact that in western societies, which are not governed by Islamic law, youngsters as young as 12 years old commit sexual acts/fornication. Which is why in turn there is a call for sex education in schools for kids in primary school to teach them about the dangers. These dangers would be nonexistent if two young adults could marry and not fiddle with others.
There are a lot of other things I could say but I don't want to derail the thread as this is a topic of it's own.