First of all the Bible teaches that restitution is an integral part of one's repentance. The Oxford English Dictionary defines 'Restitution' as a process of restoring things to its original state. In the context of the bible, I would rather word it as a process of 'restoring things to a state which is right with God'. A few Scriptures to support this would include the following...

Eph 4:28
28 He who has been stealing must steal no longer, but must work, doing something useful with his own hands, that he may have something to share with those in need. (NIV)

Take note that a person must stop sinning before he can restitute.

Exod 22:1
1 "If a man steals an ox or a sheep and slaughters it or sells it, he must pay back five head of cattle for the ox and four sheep for the sheep. (NIV)

Luke 19:8
8 But Zacchaeus stood up and said to the Lord, "Look, Lord! Here and now I give half of my possessions to the poor, and if I have cheated anybody out of anything, I will pay back four times the amount." (NIV)

Sad to say, restitution is an oft-forgotten teaching in churches today. Notice from the verse above (Luke 19:8), that restitution was part and parcel of Zacchaeus' salvation. Nobody forced him to give up his ill-gotten gains, but he himself volunteered it in front of the Lord. Today, we are seeing a disproportionately small amount of restitution in comparison with the large number of souls saved. Is restitution really that optional after all? Does it mean that a bank robber who has a loot of one million dollars gets to keep the money when he comes to Christ? So what about the man who married more than one wife before he came to Christ? Would'nt the arguments above apply to him also?

We are always delighted to hear the doctrine of restitution preached and taught in the Churches. It is a fundamental part of the Gospel of Jesus Christ and, as you rightly point out, all too neglected by the Churches. It is worth pointing out, however, that sometimes restitution is beyond the power of an individual to fulfil and he must simply do what he can. It may be that the one he has wronged will forgive him.

God's will for man in particular and mankind in general is to have only one wife...

Gen 2:18
18 The LORD God said, "It is not good for the man to be alone. I will make a helper suitable for him." (NIV)

Notice a helper not helpers.

This is reading more into the text than is actually there. It is not logical to conclude that in Gen.2:18 we are given an absolute standard of monogamy even though one may assume that such lies behind it. We must let the Scriptures speak for themselves. All we can deduce from this passage is that Adam was alone and God wished him to have a companion. There is nowhere any suggestion that Adam could not, at some future date, have more companions. We may, of course, observe that the Bible records only one wife, Eve, and deduce from that that in all likelihood Adam had only one wife. But we cannot do more than that. The option for polygamy is not excluded from the text.

Gen 2:24
24 For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh. (NIV)

Leave his father and mother and be united to his wife (NOT WIVES)!

This is stretching logic somewhat. Had the text read "united to his wives" then that would have been a divine command to practice polygamy and thus exclude monogamy. Gen.2:24 is not a commentary on whether a man should practice monogamy or polygamy but instruction on the purpose of marriage in general, namely, to become "one flesh". If this is interpreted literally, then it can only refer to the making of children, since a wife cannot unite physically with her husband to form a being like Adam before Eve was created. If it is interpreted spiritually, it can only be to be united in mind and heart, to be "one". This "oneness" can obviously be realised in both monogamy or polygamy without doing violence to the divine edict.

Matt 19:4-6
4 "Haven't you read," he replied, "that at the beginning the Creator 'made them male and female,' 5 and