What if the spouse sleeps with someone else? Can you get divorced then?
There is only one Biblical (New Testament) ground for divorce, and that is if the initial attempted marriage was unlawful. The word for "unlawful intercourse" in Greek is "porneia".
Matthew 5:32
But I say to you, whoever divorces his wife, unless the marriage is unlawful (porneia), causes her to commit adultery, and whoever marries a divorced woman commits adultery.
Matthew 19:9
I say to you, whoever divorces his wife, unless the marriage is unlawful (porneia), and marries another commits adultery.
Mark 10:11-12
He said to them, “Whoever divorces his wife and marries another commits adultery against her; and if she divorces her husband and marries another, she commits adultery.”
Luke 16:18
“Everyone who divorces his wife and marries another commits adultery, and the one who marries a woman divorced from her husband commits adultery.
That was Jesus. So we're stuck with it.