It’s because people have an incorrect/insufficient understanding of what (WHO) love is. God is love, and His love is** free, total, and life-giving**. Human love expressed sexually is supposed to be a microcosm of God’s love for us. If any sexual union does not encompass all three of these aspects, it is a perversion of real, authentic love.
Premarital sex (or, more appropriately, fornication) may be freely given to each other, but most often it is not total (because there is a lack of permanent commitment to one another; total means complete, without reservation), and is also most often not life-giving (as the couple usually uses some form of contraception). Ergo, it is a perversion of authentic love. That is the Catholic position.
The secular argument against fornication is actually the platform by which the Catholic position springboards from. It really is this simple: the sexual act exists (evolved) for a reproductive purpose. That it is pleasurable is merely a fortunate by-product for us, in the same way that foods taste good (we need food, but it doesn’t have to taste good).
In short, sex is not about love, it’s about making babies(although, of course, sex can be an expression of love, but sex is not love). Babies need a lot of time, care, resources, love, and responsibility, and parenting is not supposed to be a one person job. Children also need a present, involved mother and father for their psychological well-being. Only in the bonds of committed matrimony will this scenario be satisfied for a child.
Therefore, since 1) sex is for making babies; 2) no contraceptive method is 100% effective (except abstinence, which isn’t really a contraceptive method); 3) children are a major responsibility; and 4) fornicating couples neither currently desire nor are prepared to assume the long-term financial, moral, and social responsibility involved in having a child, then THEY SHOULDN’T BE HAVING SEX!!!