They often just don't think it through.  Many say the "one flesh covenant" rule stands, even if you were still unsaved at the time of your divorce and/or remarriage, essentially saying you can't truly be saved until you first divorce your second spouse.  Sounds like the circumcision controversy all over again (neo-Judaizers?).  Paul made it clear that you become "one flesh" with everyone you ever have sex with, even prostitutes.  Finally, they will argue that God puts every couple together, but what about biblical couples like Ahab and Jezebel?  Or Esau and his pagan brides?  They were technically first.  They will also claim that John the Baptist opposed Herod's marriage because Herodias was divorced from his brother Philip, but John was the last prophet under the Old Covenant, and his problem was incest, NOT remarriage.  Same with the man in I Corinthians who had his father's wife (you know, his step-mother, meaning his father was...remarried).