What is better: yes / no radio, or simple checkbox?

In a very big form on my company's website, there's this tendency to use Yes / No radio buttons combination.

I can think of one argument of not using it, but it is not UX related: we need to maintain three states instead of just two: null, true and false for those fields.

Is there any argument, UX-wise, to use the checkbox instead?

Edit: We do not need to cover all 3 cases, the null case is just the initial state and it is impossible to go further in the form without selecting either "Yes" or "No", hence the maintenance difficulty mention.