What An-Na'im did is, he took the rule without looking into details, he did not examine the case were both spouses are Christian or any other religion, and then one of them converted to Islam.
If the wife became Muslim, there should be a waiting period which is for the husband to think of Islam. If the husband converted to Islam before the deadline, they continue their life as a husband & wife. But if the husband does not want to be a Muslim and the waiting period finished, they should apart from each other. However, if the husband converted to Islam after the deadline, they can be together after renewing their marriage contract . We can find loads of cases in the Islamic history for this rule , for instance: the prophet's eldest daughter Zynab was married before the introduction of Islam, and after she became a Muslim, Zynab and her husband separated from each other, because her husband did not believe in Islam during the waiting period. After six years her husband converted to Islam and they returned to each other after they renew their marriage contract .