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Under the common law, a Contract could only exist if both parties had offered something of financial value, however small. This something was, and is, referred to a the Consideration for the contract. However, the courts of Equity have traditionally recognized things other than money as being of value, and historically one of the most important of these was the consideration of marriage. If X, a wealthy landowner, offers Y a house on the condition that Y marry X's daughter, then Y is offering 'marriage consideration' to X. Similarly, if X agrees to create a Trust of certain property in favour of Y, on the same grounds, then a court of equity would have compelled X to consititute the trust if he reneged. It is not clear (at least to me) whether the courts would have enforced other kinds of agreement (not land or trust agreements) on the basis of marriage consideration, or whether marriage consideration would still be acceptable to a modern court. There isn't a whole lot of recent caselaw on the subject.
Nevertheless, the existance of marriage consideration has technical implications for UnregisteredConveyancing. In particular, a person who takes an interest in land for marriage consideration will take it free of any unregistered interests that fall into classes C(i)-(iii) of the land charges act (1972). These interests include apuisne mortgage, for example. What this means is that if X borrows money under a puisne mortgage, and the lender forgets to register his mortgage as a land charge, then if X transfers the mortgaged land to Y as a 'purchaser for value', which includes a purchaser for marriage, then Y takes priority over the lender. The mortgage will not be enforceable against Y, which will be of some comfort to him as he settles down to a life of married bliss in his new home.
However, aland charge in class C(iv) or D, if not properly registered, does not become unenforceable against a person who has offered marriage consideration. It is only unenforceable the 'purchaser of the legal estate for money or money's worth', and that does not include marriage. So, if X contracted to sell his house to Z, and then in breach of contract gave it to Y, on condition that Y marry his (X's) daughter, then Z can turf Y and his wife out of their new home. However, had Y offered money, not marriage, then he would take priority over Z, leaving Z to sue Y forbreach of contract. Similarly, the bridegroom will find himself bound by any unregistered restrictive convenants and equitable easements that affect his land, whether or not they are registered.
In short, marriage is recognized as consideration in land transactions, but is a lesser kind of consideration than hard cash.