Premarital sex is still considered fornication and is a mortal sin against the sixth Commandment.
A couple that is truly in love wants what is best for each other individually and as a couple. So for example, a couple in love wants both partners to be at their optimum health, and they will mutually avoid what is harmful and dangerous to their health.
Morally speaking, sin is a disease of the soul, so a couple truly in love should want neither party to willingly and knowingly engage in sinful activity, for the sake of their spiritual health and well being.
Sex before marriage is not an act of love—it is an act of selfishness since there is no full or final commitment as there is in marriage. Sex before or outside of marriage cheapens the sacred act of covenantal love into a merely biological process to produce pleasure.
It in essence becomes mutual masturbation, since the sex act becomes more important than the person with whom one is having sex. Why jeopardize the spiritual and moral health of someone you claim to love just so you can sleep with them? Making sacrifices is what people in love do for each other. Parents sacrifice for their children.
If one or both persons in a relationship are not willing to sacrifice and wait until the wedding night to have legitimate sexual relations, then they are saying that the relationship and the other person are not worth the effort to sacrifice and abstain.