Is the virgin birth a cover story?

Hostile readers assume the account of the virgin birth is a cover story for a prenuptial scandal. That makes sense if you reject miracles out of hand, as well as the larger context of Christ's extraordinary life and ministry. 

However, even on naturalistic grounds, why would Mary or early Christian propagandists concoct a story like that? To begin with, no one except Christians is going to believe it. So it will fail to silence suspicion and allegation. The very audience that assumed the worst in the first place will hardly be persuaded by this explanation. 

In addition, it's not even the most plausible naturalistic explanation. The Mosaic law has a loophole for rape victims. If a virgin says she was raped when she was out in the field, she can't be prosecuted since there were no witnesses to confirm whether it was consensual or not (Deut 22:25-27). But that would make it harder to enforce the law on adultery, since even if a betrothed virgin (or married woman) became pregnant through consensual sex, she would always claim rape. Say she wasn't within earshot of any witnesses at the time. 

Assuming, for the sake of argument, that Mary was pregnant because she had consensual premarital sex, why make up a story about angelic visitations and a miraculous conception when she could simply say she was a rape victim? 

Rape was probably not uncommon back then. So unlike the virgin birth, there'd be no air of unreality to the claim. People who scoffed at the virgin birth wouldn't be in a position to scoff at that explanation.

Given how easy it would be to invoke this loophole, it stands to reason that some women who were guilty of consensual premarital or extramarital sex evaded the allegation by claiming to be rape victims. So long as they weren't caught in the act, there'd be no presumption that their claim was false. 

Yet Mary doesn't say that. Matthew and Luke don't represent Mary having said that. 

If you're going to invent or circulate a cover story, that would be far more plausible to hostile readers than the virgin birth. So why didn't Mary, Matthew, and Luke resort to that explanation rather than the virgin birth? For the obvious reason that the tradition of the virgin birth was the true explanation, even though it will invite derision in a way that feigning rape would not.