You have to keep in mind that you should always follow Apple's UI guidlines when you're developing your user interfaces. Apple has the best UX & UI engineers in the world, and yet, they opted to leave out such a checkbox control from their framework. Not only does using Apple's recommendations provide your users with the best user experience, it also provides them with controls they're familiar with.
You can think of what you want as a radio-button like control (either Accept or Decline). The way Apple implements radio buttons in their applications is by using the UISegmentedControl (e.g.: in mail setting when you choose between IMAP & POP).
Hope this helps