Going through the process of memorizing scripture with emphasis in verse 14 of 1 Corinthians 6. Instructions to combat those who believe sex outside of marriage is ok. If someone does believe something isn't written for them (Reference Hebrews 13:1-6 NKJV, focus in verse 4), then they do not believe the whole bible is true. Which is ok for them. But they are taking their chances on the last day when they see the Lord face to face (reference James 4:11-12 NKJV). And there is no need for you to argue with such a person.