R
It's simple. If they got a divorce that had no real grounds they are still married to their first spouse in the eyes of God so even if they "remarry" they are still married to their first spouse and they are not married to their second spouse in the eyes of God. What they may have is a valid civil marriage, but they do not have a valid marriage in the eyes of God. To say otherwise would make God a polygamist.
So what do you do with the millions of people who did divorce and remarry for reasons other than physical adultery? Do you take the position that the divorce was a sin that could be forgiven thereby releasing them to move forward in their lives or are they living in a continual state of sin by virtue of their remarriage? If they are living in a continuing state of sin and do not repent by asking for forgiveness and remedying the situation by leaving their current relationship are they sinning with a high hand and at some point does their very salvation come into question?
What is the remedy for millions of people in this situation? Do you honestly believe God would agree with destroying current family systems because of previous mistakes/sins? I don't believe that is consistent with God's design. If the parties divorced for reasons which do not fall under the umbrella of adultery than they have sinned and need to ask for forgiveness. The consequences for that sin undoubtedly plays out in a myriad of ways in their lives and relationships. The repentance piece, however, may be in the form of grasping a deeper understanding of what marriage is and what is required rather than inflicting even more pain and suffering on people including children who have probably already been through the destruction of their family once.