According to God, who may remarry?

And I say unto you, Whosoever shall put away his wife [woman], except for fornication, and shall marry another, committeth adultery: and he that marrieth her when she is put away [fornicator] committeth adultery. Matthew 19:9 ASV

Adulterers and fornicators may not remarry

Whoever puts away his spouse in order to be with another spouse except for fornication commits adultery. The problem with adultery and fornication is that both the adulterer and the fornicator will not be saved because they will not be accepted in the kingdom of God.

1 Corinthians 6:9-11 ASV
(9) Or know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with men,
(10) nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God.
(11) And such were some of you: but ye were washed, but ye were sanctified [made holy], but ye were justified in the name of the Lord Jesus Christ, and in the Spirit of our God.

Matthew 19:9 says that whoever takes the woman (marries) who was put away (divorced) because of fornication commits adultery. So, a fornicator that was formerly in a relationship with another man may not remarry as she makes the man who would want to take her an adulterer.

Fornication is detailed in Leviticus 18 from the understanding of 1 Corinthians 5 which describes the relationship of the stepmother and the stepson not as adultery but as fornication, even though the stepson is with the woman of another man except that the other man is his father. 

The Bible defines physical adultery as sex with the woman of another man like a neighbor or a stranger not a man within the immediate family.

Obviously, if the father puts away the stepmother and the stepmother decides to be with the stepson, their relationship makes them fornicators who will not be part of the kingdom of God.

1 Corinthians 6:18 ASV
(18) Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.

If they repented and fled fornication to become holy as we are called to be in Leviticus 20:7-8, they will be accepted in the Kingdom of God.

The question is, 'may the fornicators continue in life to be with another spouse?'

May the father forgive his woman who fornicated with his son and keep staying with her or must he let her go because she fornicated? May the father share his woman with his son for any amount of time because he loves her very much? In other words, do we partake the sin (s) of our spouse?

Exodus 32:32-33 ASV
(32) Yet now, if thou wilt forgive their sin--; and if not, blot me, I pray thee, out of thy book which thou hast written.
(33) And Jehovah said unto Moses, Whosoever hath sinned against me, him will I blot out of my book.

If we take the fornicator who was put away because she fornicated with her stepson we do commit adultery according to Matthew 19:9. Because of her potential to turn any man into an adulterer, if she and any other man want to be accepted in the kingdom of God, she will not enter in a relationship telling the men who are interested in her that she could give them a chance but they will both lose their salvation.

The fornication of 1 Corinthians 5 is plainly understandable on what action to take upon repentance.

What about the other sins of fornication other than sex with another member within the blood family such as step parents, uncles, aunties, grandchildren, and in-laws?

If, for example, a spouse engages in same sex relationship (fornication) out of the blood family or in zoophilia, we know that we may separate ourselves from such a fornicator spouse.

What if the fornicator repents because the fornicator wants to stay in the current relationship after they fornicated, what do we do of their repentance?

What do the fornicators of same sex relationships or zoophilia do upon repentance? May they enter into approved relationships with others or stay single forever as in the case of the fornicator of 1 Corinthians 5 involving heterosexual sins? May we take the former same sex fornicators or the former zoophilia fornicators into sexual relationships without fearing to potentially lose our own salvation?

They become a brand-new holy creature upon repentance, remission of their sin (s) and baptism (Ezekiel 33:13-20).

2 Corinthians 5:17 ASV
(17) Wherefore if any man is in Christ, he is a new creature: the old things are passed away; behold, they are become new.

May we go back to a spouse we wrongfully (not for fornication) let go in an attempt to reverse our adultery? Only if the woman did not have another relationship with another man, according to Deuteronomy 24:1-4.

Deuteronomy 24:1-4 ASV
(1) When a man taketh a wife, and marrieth her, then it shall be, if she find no favor in his eyes, because he hath found some unseemly thing in her, that he shall write her a bill of divorcement, and give it in her hand, and send her out of his house.
(2) And when she is departed out of his house, she may go and be another man's wife.
(3) And if the latter husband hate her, and write her a bill of divorcement, and give it in her hand, and send her out of his house; or if the latter husband die, who took her to be his wife;
(4) her former husband, who sent her away, may not take her again to be his wife, after that she is defiled; for that is abomination before Jehovah: and thou shalt not cause the land to sin, which Jehovah thy God giveth thee for an inheritance.

If the man cannot reverse his adultery, he must stay single forever just as it is the case for the fornicator woman who turns any other man into an adulterer because she fornicated and was put away.

As it is still the case in Moses, the one who is put away without personal fault (fornication) may still engage in a sexual relationship.

Is adultery a cause for divorce or an unforgiven sin? There is no unforgiven sin (except the sin against the Holy Spirit) because we just have to stop sinning (Ezekiel 13:14-20, Luke 5:32). The adulterer must repent as any sinner must. 

However, we do not read that we may put our spouse away for adultery but for fornication (Matthew 19:9).

Whoever takes a fornicator brother or sister commits adultery.