Marriage is also a legal contract, as well as a covenent.
I felt very strongly that my husband and I should not have a prenuptial agreement. I trust that we would be fair if we had to split our assets, even though it would be much easier now since we don't have anything. However, I have heard a view that says that everyone should have a prenup because when you are planning on getting married and in love, then you will want to be fair about splitting things up and want the best for each other--as opposed to if a marriage ends in a messy and bitter divorce, no one wants to be fair. I think this argument has its merits as well.
For couples who do have assets and also have children from previous relationships, I think that prenups can and should be used to protect your children. This is the same thing as a will, basically--saying who gets what, and no one has much problems with those and we've all seen that sometimes after someone dies families are not fair about splitting things up.
An arrangement such as this--two people get married and both have assets (houses, investments, jewelry) that they bring in to the relationship--and they have children, probably needs a prenuptial agreement, that states what goes where and to whom when/if the relationship dies. A good prenuptial agreement could go like this: a widow with a sizeable income (from both her and her previous husband's efforts) wishes to remarry, but also provide for her children's inheritance. She may have a prenup drawn up so that her assets are available to her and her spouse during her lifetime, but that a set amount be placed in a trust after her death for her new husband to live off of and the bulk of her assets pass to her children. Under the law, if she did not have this legally drawn up either in the prenup or will, then all of her assets (and her former husband's) would pass to her husband after her death and her children may or may not ever see their inheritance according to the type of stepfather they have. It would be best to have a prenup instead of a will, because wills can often be contested after the death of a person by people involved and in a prenup the parties consented to the arrangement ahead of time.
A prenup is just a will for a relationship--or a will for people still living when the relationship ends, as opposed to a will for people one of whom is at least dead when the relationship ends.
--tibac