I totally disagree as why would a loving God who gave us a high sex drive and an ability to fall in love many times demand that we can only ever have sex after a ceremony of marriage and only have one sexual partner for life? Also a marriage ceremony does not make much difference to a relationship as a high percentages of marriages fail. As long as two people love each other and the sex is consensual then I see no reason why it should be considered a sin.If marriage is a sacramental act, and, if sex is an expression of that sacrament, then, yes, premarital sex is a sin, because it points us away from God, and toward ourselves.
By the way a liberal Christian is someone who doesn't take the whole Bible literally just as it is written but instead using their intelligence to decide what the Bible means. I am from the UK and here most Christians are liberal and open minded.