So, let me put forward a few propositions that should make my position on the matter clear, and then I will deal with one of the verses in particular:
1) It is just to put adulterers to death.
2) It is just to put a son to death who beats his parents.
3) The bond of marriage is sacred.
4) Evil is to be resisted to the death.
Now, the obvious come back to this is that we do not put adulterers to death, and Jesus let an adulterous woman go. So, are we, and more importantly is Jesus, acting in a morally relative manner?
I will say that being merciful necessarily means that justice has not been served. If it were just to be merciful, then mercy would be meaningless. If Jesus lets the adulterous woman go, it does not mean that she did not deserve death. It simply means that Jesus demonstrated mercy. Christians believe that this woman will meet Jesus one day as judge, and in that day she will give an account for her deeds. Until then, she has been blanketed with grace by the King himself.
Because the Lord Jesus has extended this mercy, it is now acceptable for us to live by his example. This does not mean that new laws giving the death penalty for adultery would be unjust. Nor does it mean that Christians no longer believe that adultery is a crime against our neighbor and against God. We believe that hell awaits the unrepentant, and that is far worse than death by stoning.
Now, let me deal with what I think is the most difficult of the OT passages mentioned:
Deuteronomy 22:28-29: If a man is caught in the act of raping a young woman who is not engaged, he must pay fifty pieces of silver to her father. Then he must marry the young woman because he violated her, and he will never be allowed to divorce her.
This sounds horrible, if we understand "seize" in this passage to mean rape. The NIV certainly translates it that way, and I think that is a mistake. The verb used here in the Hebrew is a different verb used in verse 25. In verse 25, the verb definitely means "to prevail over, to strengthen against." Here in verse 28, it means, "to grab, to seize, to lay hold of." I believe it is significant that the verb is changed. Further, we have an almost exact parallel to this passage in Exodus 22:15-16. I understand Deut. 22:28-29 to describe a consensual scenario as is paralleled in Exodus 22:15-16. The verb "to grab" is used here to underscore the passion of the moment and the violation that occurs in intercourse outside of marriage, not that it is forced on the woman.
I also have an explanation as to why it is okay to eat shellfish and catfish now that Jesus has been crucified and resurrected, and it has nothing to do with moral relativism either. Hopefully that is enough to get the discussion going for those who are interested.