As stated before, going and speaking to a priest about this is good advice. But I will put my 2 cents in.
The simplest answer I can think of is God calls us to be more than animals. We are called to rule our passions and not allow them to rule us.
Premarital sex is ALWAYS an act of lust. We may believe otherwise, because we LOVE our boyfriend or girlfriend; but if we truly loved them, we wouldn't do anything to put them at risk. Since Christians know that fornication is something that God hates; and sin puts us at risk of damnation, then obviously pleasure is more important than our friend's well being.
So premarital sex is always an act of selfishness.
Then you throw in the fruits of premarital sex, I.e. Abortion, STDs, children growing up without a full family, hedonism, homosexuality, men and women seen as sex objects instead of people, inappropriate contentography, rape, child abuse, and on ind on.