Why do some "Christians" claim the God is against premarital ?

The word fornication shows up many times in the bible, Old Testament and New. Fornication means what it says. It's between people not married to each other. There is a list of people who if they don't repent of certain actions, will not enter the kingdom of God. Fornicators, liars, adulterers, backbiters, s, and the list goes on. Pretty self explanatory to me. He made it clear that is acceptable only within a marriage, and that any bed except the marriage bed is "defiled" meaning unclean, unholy and not allowed. The New Testament is filled with these references. People have just chosen to ignore them, like they've ignored everything else which is moral in the bible. They might ignore it, but GOD won't.