The claim that one should have premarital sex based on everyone doing it involves both a factual and a logical error. Although many people may have premarital sex, not everyone does. That is an error of fact. Even more importantly, something isn't necessarily right just because many people do it. If someone said "torture is acceptable in many countries" or "there are lots of rapists in our town", would that make rape or torture the right thing to do?
Next, a Christian would point out that the Bible forbids sex outside of marriage. S/he might also point out that premarital sex carries with it the risk of STDs and certain types of emotional damage, because in treating people as sexual objects we don't achieve real intimacy. Increasingly, in fact, surveys suggest that the people with the happiest sexual lives are faithful married couples, and so premarital sex may fail to fulfil even its putative goal of physical pleasure.