My wife had three children before we met and has now had an affair. Is she unmarriageable and should I leave this marriage?
Unfortunately, there is insufficient information in this question for me to answer in regards to your personal situation. You did not mention whether your wife was married prior to your marriage and, if so, the reason her prior marriage ended. Therefore, I will attempt to answer your question in general terms.
"The Pharisees also came to Him, testing Him, and saying to Him, "Is it lawful for a man to divorce his wife for just any reason?" And He answered and said to them, "Have you not read that He who made them at the beginning 'made them male and female,' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate." They said to Him, "Why then did Moses command to give a certificate of divorce, and to put her away?" He said to them, "Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery" " (Matthew 19:3-9).
Paul, in his comments on marriage stated, "Now to the married I command, yet not I but the Lord: A wife is not to depart from her husband. But even if she does depart, let her remain unmarried or be reconciled to her husband. And a husband is not to divorce his wife" (I Corinthians 7:10-11). The general rule presented by Jesus and emphasized by Paul is that a marriage has always been intended by God to last the lifetime of the married couple. "For the woman who has a husband is bound by the law to her husband as long as he lives. But if the husband dies, she is released from the law of her husband. So then if, while her husband lives, she marries another man, she will be called an adulteress; but if her husband dies, she is free from that law, so that she is no adulteress, though she has married another man" (Romans 7:2-3). Divorce has never been a favored option to God. "And this is the second thing you do: You cover the altar of the LORD with tears, with weeping and crying; so He does not regard the offering anymore, nor receive it with goodwill from your hands. Yet you say, "For what reason?" Because the LORD has been witness between you and the wife of your youth, with whom you have dealt treacherously; yet she is your companion and your wife by covenant. But did He not make them one, having a remnant of the Spirit? And why one? He seeks godly offspring. Therefore take heed to your spirit, and let none deal treacherously with the wife of his youth. "For the LORD God of Israel says that He hates divorce, for it covers one's garment with violence," says the LORD of hosts. "Therefore take heed to your spirit, that you do not deal treacherously." " (Malachi 2:13-16).
Jesus points out in Matthew 19 that divorce was introduced into the law of Moses because of mankind's stubbornness. There is no requirement to divorce. Instead, there is encouragement to work at keeping the marriage together. When men insist on doing wrong, God still regulates the impact of the sin. The general regulation stated by Jesus and Paul is that if a divorce occurs, the couple is to remain unmarried or return to their former spouse. A person divorcing his spouse commits adultery when he marries another. A person who has been divorced commits adultery when he marries another. In addition, the person marrying a divorced person also commits adultery. "But I say to you that whoever divorces his wife for any reason except sexual immorality causes her to commit adultery; and whoever marries a woman who is divorced commits adultery" (Matthew 5:32).
There is one exception to this general rule regarding remarriage. If a person divorces his spouse because of fornication (sexual immorality), then that person is permitted to marry again without committing adultery. The grammar of Matthew 19 is such that the exception is given to the one divorcing his or her spouse because of fornication, but the exception is not given to the one who had committed fornication and is being divorced as a result. Since no exception is given to the one who had committed fornication, it must be concluded that their unfaithfulness to their marriage vows denies them the right to marry another.
In this age of "no-fault" divorces, this sounds harsh and restrictive. The disciples thought the same when Jesus explained to them what God desires. "His disciples said to Him, "If such is the case of the man with his wife, it is better not to marry." But He said to them, "All cannot accept this saying, but only those to whom it has been given: For there are eunuchs who were born thus from their mother's womb, and there are eunuchs who were made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the kingdom of heaven's sake. He who is able to accept it, let him accept it."" (Matthew 19:10-12). Eunuchs are men unable to have sexual intercourse. Jesus acknowledges that the teaching is tough, but there are some who will be willing to forego sexual relations in order to reach heaven. There are many who have made a mess of their lives prior to coming to the knowledge of the truth. Sometimes the only way to straighten out the mess is to make a commitment to live a life of chastity -- a difficult decision that only some will be willing to make.