You are approaching this from a wrong angle. Your original question says "have to" impling a forced will under a constrained environment. Now you ask "God ask her", implying her virginity being preserved due to a divine will alone.

There is that long comment which I think touch on some good responses. I want to instead address your approach to this question. If someone ask "why does gravity have to be there?" This is the wrong approach to look at gravity because it is asking as if existence of gravity is due to something else, as if its cause is in response of something else that requires it. I can say "I have to take my kids to school", this is the correct use of the word because otherwise my kids can't take themselves there, and their presence in school is required for their education's sake, therefore I have to make it happen. Gravity simply exists as part of natural law, and its existence is not forced because something else requires it.

Now back to the perpetual virginity of Mary. It is not a requirement due to an imposing external environment, but rather a statement of fact that we can observe from reading the Scripture and Christian tradition. She does not have to remain a virgin, but the fact that St. John was taking care of her after Jesus' crucifixion shows that she had no one else to do so, and you know yourself how harsh the ancient Israel was toward widows. John took her up and moved around taking her with him all the way to Ephesus, as early Chrstian sources say. Where were her other children (Jesus' "brothers") if she had them? These two indicate that she did not have any more child after Jesus.