This is a tough subject for a lot of Christians in a culture that accepts most sexually immoral choices. Whats even more scary is that some Christians support the wrong choice or are too fearful to rebuke it out of concern for offending or being labeled 'judgemental'. What do these adulterous choices, that are accepted and more normalized look like?
Pre-marital Sex
Part of human nature is to desire sex for a number of reasons. The Holy Spirit through Paul makes it clear that the Godly acceptable solution to fulfill this natural desire is within a marriage (1 Corinthians 7:2). Notice it is singular per individual. This verse explicitly clears up the air on a number of sexual issues. First, sexual relations are only acceptable to God within a marriage; thus, outside of marriage it is immoral. Second, a single male should be with a single woman, and a single woman with a single man. Thus, polygamous marriage is outside of God's acceptable range since this revelation (although it was accepted earlier in history for populating the earth after the flood). Third, a single male should be with a single woman, and a single woman with a single man. Thus, same-sex marriage is also outside of this declaration of what is acceptable to God. But those are different topics. Here pre-marital sex is revealed as sexually immoral and there are countless verses that discuss sexual immorality (Acts 15:20; 1 Corinthians 5:1; 6:13, 18; 10:8; 2 Corinthians 12:21; Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3; Jude 7). Waiting to have sex until marriage is honorable and keeps the relationship from becoming defiled before God (Hebrews 13:4). This is one common and normalized adulterous act in modern secularized society. Having Pre-marital sex is not in anyway God's Will or God's leading. God does NOT tempt us to sin (James 1:13) but will always give us a God Glorifying option in the midst of the temptation (1 Corinthians 10:13). He may allow the temptations, but uses our own sinful hearts or works of evil spirits (Psalm 141:4), to have triumph over them which brings him Glory (Luke 22:31-32).
Lust For Another
A growing commonality in modern relationships is the idea that maybe a man or woman married the wrong person. Then, they meet someone new, allow themselves to grow affections for them, being to entertain the thought of being with that person instead of their current spouse and POOF the idea that maybe this other person is the right person pops in their mind fed by the lustful desires of their hearts. In Matthew 5:27-28, Jesus makes the bold declaration that even looking lustfully at another person while married is adultery; without ever touching the person. Adultery can manifest in the heart before it every manifests in the flesh. The "adultery" that Jesus is specifically referring to is talking about people who are married lustfully looking outside of the marriage at another person. But this does not just apply to people you meet, but also what you see. Pornography is the same. If anyone finds themselves becoming distracted by another person outside of the marriage, they are called to take quick action and "flee" from it (1 Corinthians 6:18). Lusting for a person outside of the marriage is in no way God's Will or God's leading.
Divorce
Divorces are overly common and in some cases encouraged. Some states have a 'no fault' divorce status. Even in the church, over half of Christians get divorced. Some Christians will encourage it. Jesus in Matthew 19:9 and Matthew 5:32 says, "I tell you that anyone who divorces his wife, except for marital unfaithfulness, and marries another woman commits adultery." With the exception clause aside, remarriage is adultery. God's plan for marriage is a life long plan (Genesis 2:24; Matthew 19:6). Now discussing the 'exception clause' we see something about it as well. Understanding the issues that were taking place in that culture and why God 'allowed' divorce is important. First, wives were being beaten, kicked out of the house, and even killed because the husband wanted a new wife or the wife would not bare children. Moses allowed it to save the woman's life (Matthew 19:7). But Jesus makes it clear, it is because of 'the hardness' of the hearts and NOT because of God's plan or will (Matthew 19:8-9). Thus; divorce, even though in slim allowances, was still not part of God's plan for marriage. In other words, God will not lead someone into seeking a divorce. If God can forgive us for all our monstrous sins, husbands and wives need to first try and forgive sins against each other. Interestingly, following the discussion of divorce, Jesus called the children to him and prayed for them (Matthew 19:13). We all know divorces negatively effect children. And it seems that Jesus is implying that divorce can hinder children from coming to him (Matthew 19:14).
It also seems that divorcing an unbeliever is allowed (but not part of the original plan) in 1 Corinthians 7:12–15. Why, in Paul's opinion (1 Corinthians 7:12), does he feel this was allowable? Even though God does not like divorce (Malachi 2:16), unbelievers are actually spiritual adulterers to begin with. People who do not believe in God are seen by God as committing adultery in a spiritual context (Jeremiah 3:20). And loving the world more than God is just that (James 4:4–5). Thus, they are living an immoral life a part from God, allowing for the exception clause. Divorces outside of what is accepted by God puts people in a constant state of adultery. Living in an unrepentant state of sin. Divorce within the exceptions allows for repentance and redemption.
Singleness
Sometimes it is best for divorcees to remain single. Singleness is NOT a bad thing. In fact, it allows for more person time with the Lord (1 Corinthians 7:7-8). Singleness is an opportunity to serve God wholeheartedly and undistracted (1 Corinthians 7:32-36). BUT if someone 'burns with passion' and has a need to be in a relationship then it is those people who need to remarry (if divorced by the exception clause). If someone just leaves their marriage for any reason except the clauses, then it is best for the not to remarry for the sake of not being in perpetual sin. They should seek the Lord and pray for his help in avoiding future sexual sins and seek reunification with their spouse. In Matthew 19:12 there are people born or made unable to have sexual relations; they bring God glory as well.
Abuse
Abuse is not to be tolerated in a marriage either. Abuse is contrary to a Godly marriage, immoral, and illegal. The proper authorities should be contacted in the event it takes place without question. The Spouse and children need to be kept safe no matter what. But we need to understand something, there are habitual abusers and those who are not and make a huge dangerous mistake. No where in scripture does God command divorce. God does allow divorce but it should never be the first option. The worst marital advice is to leave the marriage without seeking restoration and redemption. This can be done through mediation and supervised counseling over time. The abuser and victim, if believers, needs to seek the Lord first and foremost. The abuser needs to seek God's mercy and genuinely repent. The victim needs God's love and assistance in forgiving. The church needs to support both. If the abuser demonstrates verifiable change, independently confirmed, the relationship may be resumed with much caution. The church needs to follow up with the couple constantly and be a support mechanism to foster relational and spiritual growth. We must all keep in mind that Christ was abused until death, yet, he remained humble, loving, faithful, and sought the absolute best for the abusers; who is you and I.
Conclusion
It is pretty much blasphemous to say it was God's will a person leaves their marriage for someone else they fell in love with while married. Pre-marital sex, no fault divorce, and divorce because someone simply 'fell out of love' is adultery. It is God's Plan for marriage that the spouses fight for each other in a life long battle for each other. Grace, love, and faithfulness does not depend on the other person. But in the event, a person becomes unfaithful or is an unbeliever, AND genuine attempts to restore the marriage fail; then divorce is permissible. We must always recognize that each person in the marriage is a sinner, and will sin. At the end of it all, no matter the modern cultural norms of the simplicity of divorce and normalcy of remarriage; God is clear and adultery is adultery even if it is dumbed down with words like 'a mistaken affair'. It is and forever will be sexual immorality completely independent from social norms.