For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Adultery is the only grounds for divorce biblically
Does your church have any type of care program? I attended divorce care sessions. It helped me understand my position and Gods intentions for marriage and divorce. You do not have to be divorced or separated to attend. It may help you in your decision and how to handle your choices and direction.
One of the things that makes this doubly hard to deal with is all of the sexually transmitted disease that is out there. The one cheating could transmit it to their mate. All sin has a reward. Even when God does not chasten us right away. We still reap what we sow. I do believe God hates adultry far more than almost all other sins.
The husband wife relationship is much like our relationship to God. If a man repents and turns back to his wife and to God, he should be forgiven. But as Paul said, if there is no repentance, then he should be turned over to satan to kill his body so that his spirit will be saved in the day of the Lord.
God takes our marriage vows as important as our relationship to Him. Adultry affects more than one person. If there are children it can do great harm to them. It also affects the lives of those around them like the parents of the bad marriage. Almost all sin affects more than one person, and this one is huge in Gods eyes.
Sorry to say it is rampant even among christians today and it destroys many people.
Having said all of that, while I do not believe a born again believer can lose their salvation, I sure do believe they will lose their reward. Some will be saved so as by fire and that shows the power of God to save even the worst of sinners.