How can I assign values to radio buttons with the forms api?
The keys of the array set as #options determine the values and the values determine the labels.

In other words: The example in my question uses a regular array array(t('M'), t('F')) which, behind the scenes, is an associative array using non-negative integers as keys (same as array(0=&gt;t('M'), 1=&gt;t('F')). Since the radio buttons use these keys to know what to store in the database, the solution is simply to change them: array('M'=&gt;t('M'), 'F'=&gt;t('F')).