Bismillahi Rahmani Rahim - In the Name of Allah, the Beneficent, the Merciful
It is true that Islam does not allow a Muslim woman to marry a non-Muslim man, and that in the instance of a woman converting to Islam, her marriage will become invalid after a period of time if her husband chooses not to convert.
The wisdom of this ruling is that in Islam the man is recognized as the head of household, and under normal circumstances he will most likely have a greater influence on allowing or not allow his wife to practice her religion. Also, he has a great say on how the children will be raised. Rather than risk having the woman pressured to abandon her faith, she is instead given a length of time to talk to him, have others talk to him, and to lovingly encourage him to consider Islam before she is separated from him. The position in Islam is clear: Islam is the truth and it is the clear path to Jannah, and it is more important to avoid having a woman abandon her faith than it is to preserve the marriage. If the husband is guided to Islam through her that is a great blessing. If not, then Allah will give her something better. The short-sighted only see the disruption this situation causes. Those with firm belief in the Hereafter recognize that sometimes upheaval is necessary to allow the woman to practice her religion. That being said, if a woman sincerely converts to Islam and she cannot bring herself to leave her husband due to her love for him, it is still better to be in this situation rather than not to convert to Islam. Perhaps over time she will influence him to convert as well. And Allah knows best.
Fi Aman Allah,
Nancy Umm Abdel Hamid