I have had sex with the girl I proposed to marry. Now we are very close to our wedding. Her pastor became aware that we have had sex by virtue of her confession. She was told that I am not her husband and as such, can not get married since we slept together.
Sir, I need your candid advice.
Also, I want find out: if people who have prayed and are convinced that God approved of their marriage and later fall into sex before wedding, does it invalidate the fact that God spoke to them to be married? Can God disapprove the marriage confirmed by Him before because of sex after confession and repentance have taken place?
Let me start out by being very clear: sex outside of marriage is wrong. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). I get the impression from your note that while you see it technically as wrong, you don't think that what you did was all that bad because you planned to get married anyway. But your plans did not change the fact that you sinned.
It is correct that you are currently not her husband. Sex does not create a marriage. It is the wedding vows, the covenant, that creates a marriage. "Yet she is your companion and your wife by covenant" (Malachi 2:14).
However, the preacher is wrong. There is no law forbidding fornicators from getting married. Rather you find the opposite: "But I say to the unmarried and to the widows: It is good for them if they remain even as I am; but if they cannot exercise self-control, let them marry. For it is better to marry than to burn with passion" (I Corinthians 7:8-9). If this man refuses to marry you, then find someone who will.
Prophecy ended nearly 2,000 years ago when the Bible, the perfect law of liberty, was completed. "Love never fails; but if there are gifts of prophecy, they will be done away; if there are tongues, they will cease; if there is knowledge, it will be done away. For we know in part and we prophesy in part; but when the perfect comes, the partial will be done away" (I Corinthians 13:8-10). God speaks to us today through that perfect law. The idea that God directly tells people what choices to make is false. God told you through the Scriptures how to pick a good spouse. You believe you have found such a person. Then go get married as God commanded and stop the sinning.