If an unmarried couple has sex, are they married in God's eyes?

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.