Most of what muslims believe about Mary is found in surah 19 of the Qur’an. In surah 19, there is nothing there that says that Mary remained a virgin-- it only says that she was a virgin when she was impregnated. Surah 19:20 says “She said: "How shall I have a son, seeing that no man has touched me, and I am not unchaste?”. Islam does not affirm or deny that Mary was perpetually a virgin.
As for your question about Fatima, I’ve never heard that theory before; and I was a muslim for six years. Her husband, Ali ibn abu Talib, was the fourth caliph of Islam. He was one of the most influential muslims of his generation, so if his wife had become a christian, certainly he would have said something about it. There’s nothing in sunni literature that says that she became a christian. Even in shi’a sources, as far as I know, there is no evidence that she became a christian. Believe me, if she did, the sources would be full of evidence about it and scholars would have been talking about it for years to come (people such as Imam Bukhari, who authored the hadiths of Sahih al-Bukhari, which is the most authentic collection of sunni oral traditions). Imam Bukhari didn’t mention it, so we can be fairly certain that Fatima did not become a christian.
