Pre-marital sex is considered a mortal sin by the Catholic Church. In other words, it is a sin so grave that if you do not genuinely ask forgiveness for it, it could doom you to hell. Here are some of the reasons why:
1) Human law provides the minimum standard for humans to live by. In other words, what is the most that humans can get away with without destroying themselves? God's law is exactly the opposite. God holds us to the highest standard possible. In other words, what behavior would be needed in order to achieve an absolute perfect society? We don't change God's law to suit us because we fall short. We don't even change God's law because it's impossible for humans to achieve. Instead, we adapt because we should always be striving for the ideal. With that stated, what kind of behavior do you think God would expect of unmarried couples?
2) Moral behavior also requires that we have the highest respect for human life possible. As such, we should never do harm, short of self-defense. Pre-marital sex, no matter how careful, always has a risk of pregnancy. This could lead the mother to another mortal sin through abortion. The couple is also putting a potential human life's entire future at risk by not being prepared to accept the responsibility to raise it or be prepared to do so in advance. It might have to grow up in a single parent or unstable household, finances would not have been prepared, and so on. The fact that a couple could potentially put an innocent life at risk is the highest form of selfishness and is completely irresponsible.
3) Despite years of scientific study and philosophical debate, no one knows precisely when the soul enters the human body and human life begins. As such, Catholics are required to take the safest route and assume the most conservative answer, which is that life begins at the moment of conception. Birth control can cause the failure of a fertilized egg to implant, thus causing an abortion. Someone could be the unknowing partner to unintentional murder. Some might also choose to use the morning after pill, which does essentially the same thing, but on purpose.
4) Pre-marital sex always presents the possibility of sexually transmitted disease. It would be a grave sin to infect someone with a dangerous disease, and some diseases, like HIV, can be transmitted even through condoms. People might not even know they have them. HPV has no symptoms, and yet can cause cervical cancer. How many times have people believed themselves to be in a monogamous relationship, only to discover that their partner is cheating? Over 50% of the population has been cheated on at some point. How many had suspicions beforehand?
5) Promiscuity leads to more promiscuity. By sleeping around, it decreases the special bond that one has when they are exclusive with a single partner. They want to explore the lustful side of sexuality more, unimpeded by feelings. It increases the possibility of cheating during marriage, and has helped to contribute to our skyrocketing divorce rate. Why get married when I can just sleep with someone and move on? People get obsessed with the initial sexual attraction, and never move on to the more intimate phases of relationships. When the fireworks stop, they simply dump their lover, find someone new, and recapture the feeling again. You always hear in the movies about "fear of commitment". Well, a LOT of this is due to the fact that people think they have a good thing going. Why would they want to limit their sexual fun by committing permanently to someone?
6) During pre-marital sex, one is treating the partner like an object of pleasure, rather than an object of love. (This can occur EVEN INSIDE OF A MARRIAGE, by the way...) Rather than using sex for its wondrous purpose, which is a special unity that offers the possibility for the creation of life, we detach from that purpose, and use it solely for pleasure. To be sure, sex is pleasurable and provides fun and great intimacy for couples, but it is meant to be a unique bond that strengthens the family unit, rather than fracturing it through promiscuity. By giving into lust, a couple is exploiting each other for selfish purposes, even if the bond of love is there. The minute that sex becomes just about fun, the couple begins to test those boundaries to see how much fun one can have. These boundaries are difficult to resist, and one can find themselves in trouble very fast. Even inside a marriage, when you are not open to life, you are taking one of the most special gifts of God, which is actually being allowed to participate in the process of creation, and rejecting it as if it is worthless.
So, why is pre-marital sex bad? It could lead to abortion, it could lead to the creation of a child with a family completely unprepared to raise it, it can transmit disease, it is selfish, it weakens the family, it provides a mentality for divorce and lack of commitment, it is exploitative, and is the misuse of a hugely unique gift from God. Does that help?