There are many reasons to consider adopting a child. Perhaps you and your spouse cannot have kids naturally. Maybe you want a child but you don't want to go through the physical difficulty of pregnancy. Perhaps you're a step-parent and you're thinking of adopting your spouse's child from a previous relationship.
People often set themselves firmly against using a prenup on the grounds that it just feels too cynical. They're in love. They never plan to get divorced. They don't want to think about that.