Adultery is a sin against God and against one's mate:
"Let marriage be honorable among all, and the marriage bed be without defilement, for God will judge fornicators and adulterers." (Hebrews 13:4)
Sex relations outside the marriage is the only Scriptural ground for divorce allowing for remarriage:
"A married woman is bound by law to her husband while he is alive; but if her husband dies, she is discharged from the law of her husband. So, then, while her husband is living, she would be styled an adulteress if she became another man’s. But if her husband dies, she is free from his law, so that she is not an adulteress if she becomes another man’s. (Romans 7:2, 3)
Jesus acknowledged that fornication is an acceptable (and only) ground for a Scriptural divorce:
"Whoever divorces his wife, except on the ground of fornication, and marries another commits adultery." (Matthew 19:9)
The Bible doesn't encourage separation as a way to solve minor problems. (1 Corinthians 7:12, 13) However, if a spouse poses a threat to their mate's health or life, or their extreme opposition makes it impossible to worship God, then they may reasonably choose to separate.
For more, see:
Does God Hate ALL Divorce?
The Option of DIVORCE
Family Life That Pleases God - What is the Bible's view of separation and of divorce? (6, 7)
Marriage / Family Life
Search For Bible Truths - ARCHIVE
Search For Bible Truths - Search Guide