Marriage Contracts
Marriage Contracts, while looked down on by many FanFiction authors, are not as unlikely as you might expect. In point of fact, they are quite common in the US where they are called pre-nuptial agreements. They are a common legal step taken before marriage. A prenup establishes the property and financial rights of each spouse in the event of a divorce, as well as the disposition and responsibility for any children born during the marriage. They can also be used to define marital responsibilities.
In an environment such as that described in Harry Potter, it would not be unlikely that such agreements would include inheritance rights for the children of the marriage. Or that such contracts might include the possibility of divorce should children not be conceived.
As for divorce, I see statements in many stories that marriage is forever. However, J.K.R. mentions in the books that Celestina Warbeck married three times and divorced two times. So, marriage, like in the Muggle world, need not be a forever situation.
As for more onerous conditions in fan-fiction Marriage Contracts, considering that the Victorian Era laws considered wives and children to be mere property, incapable of owing property in their own right, to be used and disposed of as needed by the husband, such conditions are not unreasonable. Others in the Victorian Era might go "tsk, tsk," but a man could beat his wife or child to death and no one would say he didn't have that right.