2 arguments against sex outside of marriage
All Christians would agree that adultery is wrong and that they will be judged by God for breaking His commandments. It breaks the promises that Christians couples make before God during their wedding, to forsake all others and be faithful to each other.
Most humanists and atheists would agree that sex outside marriage would be wrong. This is because it involves breaking trust, commitment and promises made, as well as the emotional consequences of adultery (e.g., for children the couple had).