new_case_form.html.erb

<%= simple_form_for(@case) %>
  ...
  <!-- in your fields for one of the spouses -->
  <%= f.input :title, collection: User.title_options %>
  ...
<% end %>

user_with_a_constant.rb

class User
  # create a constant which is only defined one time
  TITLE_OPTIONS = ["Mr.", "Mrs.", "Ms."]
  
  # to avoid having to adapt to changes in the 
  # way we retrieve the list of options, 
  # create a method on the User class itself which returns the constant
  # (this is different from an instance method)
  def self.title_options
    TITLE_OPTIONS
  end
end