Reading a bit or Corinthians last night I found something interesting, specifically on sexual immorality and judging. Paul mentions that he isn't too concerned about sexual immorality outside the church, as it isn't us, but God is who will judge their actions. Therefore, as Christians we probably ought not be too concerned for the actions of sexual immorality outside the church either, to the point that they do not affect us morally. We should protect our own, discerning of right and wrong, and seeking to be light to those outside the church, yes. But judgemental, no.
It sounds like a fairly libertarian policy to me. As such, I would prefer that the government simply remove itself from the marriage business and stick only to contractual business in this area, line of inheritance and the such. This may be simplified, but hopefully you get the point. As you stated, social engineering is not a business the government ought to be involved in.
On the topic of marriage being binding for the lives of the participants, well, as a Christian it seems only natural that it would be the case. From the garden, God instituted the marriage relationship to be for a lifetime. The marriage relationship is a reflection of Christ and his church, ever faithful to his people. While some might (as above) claim that polygamy was common in the OT, and therefore conflicting with Christianity, they would be misunderstanding the purpose of the OT. The Bible, especially the OT histories are not about good people and all their good works. They are histories, and one of the key points is that Israel, even a people chosen by God and blessed by God cannot live up to the perfection requirements of the law and that the need for a savior exists. Christ addresses some of the issues of divorce in a number of passages, and explains that divorce was never the plan, but granted by Moses because of the hardness of the hearts of the people. Sadly in a fallen world there will be pain in so many relationships, but Christianity is about reconciliation. Most importantly with God, but also with our fellow man.