We may be familiar with the teachings of Christ about divorce, and that a person may not divorce his wife for as long as he lives otherwise it is adultery if he or she marries another. But Christ's teachings also apply to fornication.
In our society it is considered acceptable to have had a number of sexual partners before marriage. This way of thinking is also prevalent within the church, in which case they are called "mistakes ", forgiven as easily as telling a white lie. But in the bible losing your virginity is far more serious than a simple mistake like telling a lie. How do we think Adam and Eve were married?
There was no priest to do a ceremony and say “I now pronounce you man and wife" and sign a piece of paper. They became married by the first act of intercourse!
Here is the point: a person is married in God's eyes to the person they first have sex with. The bible teaches that the two become one flesh. Blood seals covenants, and the flow of blood seals the marriage covenant with that person for life. As Jesus taught, God joins two people together through the act of intercourse (Not when a priest says “I now pronounce you husband and wife" in a marriage ceremony!
Mark 10: 8 And the two of them shall be one flesh. So then they are no longer two, but one flesh.
Mark 10: 9 Therefore what God has joined together, let not man put apart.
Why is this an issue? Because any subsequent sexual partners (whether you legally marry them or not) are in reality committing adultery as you are already married to your first sexual partner in God's eyes. The bible teaches that if a man has sex with a virgin he is obligated to marry her and not divorce her for the rest of his life, whether he or she likes it or not (see scriptures below).
Exo 22: 16 “If a man seduces a virgin who is not engaged, he must pay the bride price for her and marry her.
Deut 22: 28 If a man finds a girl , a virgin not engaged , and lays hold on her , and lies with her , and they are found, 29 - then the man who lay with her shall give to the girl’s father fifty shekels of silver, and she shall be his wife. Because he has humbled her, he may not put her away all his days.
Paul also stressed the importance of marrying a virgin:
1Co 7 :36 But if anyone thinks it behaving himself indecently toward his virgin (if she is past her prime, and so it ought to be ) let him do what he will ; he does not sin; let them marry.
It also taught that you become one flesh with the person you sleep with, even if it is a prostitute:
1Co 6 :16 Or do you not know that he being joined to a harlot is one body ? For He says, the two shall be one flesh.
Be warned - in the bible there is no difference whether the first time 'meant anything ' or not, whether it was a moment of weakness or planned intent, and there is no difference between whether it’s done with a prostitute or a stranger or someone you are deeply in love with - the person you first have sex with, you are married to! But I’ll get back to this later...
Now why is it important for a Christian man or woman to seek a virgin husband or wife? : The bible says that temple priests were not allowed to marry non -virgins:
Lev 21: 7. They shall not take a wife who is a LovePeddler, or defiled. Neither shall they take a woman put away from her husband. For he is holy to his God.
Eze 44: 22. Nor shall they take for their wives a widow nor her who is put away. But they shall take virgins of the seed of the house of Israel, or a widow who had a priest before.
And because Christians are priests in the service of God the same applies to Christians:
Rev 1:6 and made us kings and priests to God and His Father, to Him be glory and dominion forever and ever. Amen.
How to escape this life-long marriage to your first sexual partner?
Simply, you must die, because the only way in which the marriage covenant is broken is through death:
Rom 7:2. For the married woman was bound by law to the living husband. But if the husband is dead, she is set free from the law of her husband.
To die, you must come to Christ and be cleansed and reborn as a new creature through baptism and you re-gain your virginity, in Christ (see Romans 6),
Col 3: 3, for you died and your life has been hidden with Christ in God.
You die, you are re -born, you are made alive again, you are given new life, and so you are unmarried from your first sexual partner through death, and a chaste virgin to be married to Christ! :
Rom 7 :4 So , my brothers , you also have become dead to the law by the body of Christ so that you should be married to Another, even to Him raised from the dead, that we should bring forth fruit to God.
And because you will be married to Christ, you are a virgin again and a completely new unmarried person:
2Co 11: 2, for I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ.
2Co 5:17 So that if anyone is in Christ, that one is a new creature; old things have passed away; behold, all things have become new.

thank you @ op