It is, above all, the deepest sign of the complete gift of self that a husband and wife pledge to each other. This mutual gift empowers the couple to become co-creators with God in giving life to a new person, a baby. So sexual intimacy belongs only in marriage. Outside of marriage, sex is a lie. Before giving your body to another person, you need to give your whole life, and you need to receive your spouse's whole life in return - and that can only happen in marriage.
People can be wrong in matters of conscience, and people often are.
Here, as in everything we do, we need an objective standard to tell us if our conscience is properly formed and able to make right judgments. Conscience is God's voice, speaking the truth deep within your heart. You are acting in good conscience when you choose to do what God intends. The choice to live together outside a marriage is always wrong and sinful. Many of our family and friends are doing the same thing.