Good question Good question. I addressed this at the end of the sermon. It is a weighty question, indeed. I believe John Piper's sermon on this passage is very helpful, and I would encourage you to take a look at it. â€œWhat God Has Joined Together, Let Not Man Separate, Part 2," which can be found on desiringgod.org
I agree with Piper that a second marriage should NOT be ended. It is true, it should not have happened. But two wrongs do not make a right. The second marriage, although it was sinful to enter into, is still a real marriage. By God's grace, this second marriage can be sanctified and can reflect the relationship of Christ and the church.
Couples in a second marriage should not be told to divorce, but rather to make their marriage as Christ-honoring as it can be.