I gently advise to not blame one person for choosing wrong.
Deception is often very charming, and manipulative.
Furthermore, it is disingenious for the other party to be dating someone who regards their current legal bond so lightly. For an in depth and extremely conservative study on this subject see the book "The Divorce Myth", by J. Perhaps in an effort to avoid legalism, we have lost perspective on the fact that God does know best, and His ways are always best. Maybe a life of celibacy for the divorcee who simply stays close to Jesus is much better for them than the possibility of making the same mistakes over again in a subsequent marriage.) Perhaps we have come to the place in American Christianity where we think that because the scriptures state clearly that God is love, we don't have to reckon with His other attributes, such as wisdom, justice, etc. As to responsibilities of the church in regards to confrontation and accountability, the scriptures are clear (1 Tim. Confrontation must always be done with a view toward restoration of the repentant. Seek wisdom from your church leadership, if possible. The answer might be that in the case of "dating" there is no evidence of sexual misconduct and therefore no clear point of sin to confront.
One example is Christians living together prior to marriage; it seems that has become much more common these days.
I think David Carter did an excellent job in describing the church's responsiblity to confront these types of issues.
The responsibility does not only lay with one person to make the wise choice.
As in my circumstances as a survivor, I never would have known or imagined my soon to be ex spouse was capable of doing what was done, or the degree of sexually deviancy that surfaced only many years into the marriage.
You raised some good questions, especially about the church's responsbilitiy to hold church members/eaders to account.