I was wondering why "Christians" get Re-Married after a divorce while the husband is still alive? I passed up?

If you are married and your spouse commits adultery then you have spiritual grounds for a divorce and are free to remarry. Jesus' own words; Matthew 19:9 "I say to you that whoever divorces his wife, except on grounds of fornication, and marries another commits adultery." That of course works both ways a wife could divorce her husband and remarry if he commits adultery. Fornication is any sex outside of a marriage arrangement.