Christians make two main errors regarding divorce. One is thinking that just about every case of divorce should be given a pass without consideration of whether the causes were justified or whether there is need of repentance. The second error is thinking that divorce is rarely or never justified so that every case of remarriage constitutes adultery.
We get a better understanding if we ask, "What did Jesus actually teach on this subject?" Since the whole Bible is inspired by the Holy Spirit, we should also ask, "What does the Bible say?" Jesus clearly teaches that sexual immorality IS just grounds for divorce, saying "I tell you that anyone who divorces his wife, except for sexual immorality, and marries another commits adultery." To summarize, marriage is a covenant between husband and wife. Sexual immorality breaks that covenant thus giving the innocent spouse an option to leave and possibly find another partner or to reconcile. But this option needs to be exercised in a timely manner. It is not as if the innocent spouse can forgive and reconcile with a repentant partner but keep a long-forgiven instance of adultery in their pocket indefinitely as grounds for divorce if the marriage later becomes inconvenient. And no spouse is innocent if they've engaged in immorality also before the legal divorce is final.
Divorce is not only right, but it actually becomes advisable if the partner repeatedly returns to the sin of sexual immorality. One should not be fooled by feigned repentance and worldly sorrow. Forgiveness, mercy, and grace cannot become a license to sin without consequence. At some point, indefinite tolerance of ongoing sexual immorality amounts to tacit permission for the ongoing immorality. A spouse who gives permission for sexual immorality or intentionally closes their eyes to it is not longer innocent in the matter, since they have become complicit. Scripture says, "Let no one be found among you who is sexually immoral" and "with such a man do not even eat." Indefinite tolerance of unrepentant sexual immorality is not permitted in the Christian community, and that includes Christian homes.
Scripture is clear that unrepentant use of pornography is sexual immorality that justifies divorce. Jesus said, "Anyone who looks lustfully at a women has already committed adultery with her in his heart." The Greek word for sexual immorality that justifies divorce is porneia which is where the word porn comes from. Porn is a moral filth that breaks the marriage covenant. The community of Jesus followers, including spouses, need to stop giving purported Christians a pass on unrepentant sexual immorality in any form, including pornography. Given the prevalence of porn addiction and adultery among Christians today, it might even be that most divorces are justified.