It is true that sexual relations is the ultimate fulfillment of a couple becoming “one flesh” (Genesis 2:24). However, the act of sex does not equal marriage. If that were so, there would be no such thing as premarital sex—once a couple had sex, they would be married. The Bible calls premarital sex “fornication.” It is repeatedly condemned in Scripture along with all other forms of sexual immorality (Acts 15:20; 1 Corinthians 5:1; 6:13,18; 10:8; Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3; Jude 7). The Bible promotes abstinence before marriage as the standard of godliness. Sex before marriage is just as wrong as adultery and other forms of sexual immorality because they all involve having sex with someone other than your spouse.
If an unmarried couple has sex, does that mean they are married? The Bible gives us no reason to believe this to be the case. The act of sexual relations may have made them for a moment physically joined, but that does not mean God has joined them together as husband and wife. Sex is an important aspect of marriage, the physical act of marriage. Sex between unmarried people, though, does not equal marriage.