1. A stereotypical objection to the virgin birth is that it's only attested in two of the four Gospels. Likewise, Paul is silent on the subject.
A potential problem with stereotypical objections is how they condition people who view an issue. If an issue is routinely framed in a particular way, it may not occur to people to think outside that framework.
2. Before getting to my main point, Paul's silence is to be expected. He was an adult living in Jerusalem at the time of Christ's public ministry. It's hardly surprising that he talks about events so close to his own time and place, in the life of Christ. By contrast, the birth of Christ probably took place several years before Paul was born.
3 Apropos (1), I'd recast the issue. If anything, what's striking is not that the virgin birth wasn't recorded in more than two Gospels, but that's recorded at all. Reporting the circumstances of his conception poses a dilemma. In the nature of the case, a NT author can't mention the virgin birth without simultaneously informing his readers that Mary was pregnant out of wedlock. After all, you can't have one without the other.
But the moment he says Mary was pregnant out of wedlock, that opens a can of worms. Only people who are already Christian believe the story of the virgin birth. By contrast, people who aren't Christian are inclined to view the virgin birth as a cover story for a prenuptial scandal.
Indeed, that was Joseph's initial reaction. When he discovered that she was pregnant, he was planning to divorce her, on the assumption that she had a child by another man.
So why would Matthew and Luke record the virgin birth unless they thought it happened? You might say the reported the virgin birth despite the virgin birth. For surely they knew that by recording that story, their account invited a contrary interpretation.
By narrating the virginal conception of Christ, they were starting a fire they couldn't extinguish. Enemies of the faith will seize on that to discredit Jesus. They will say this is a transparent alibi to camouflage the fact that Mary had premarital sex. Not only would that stigmatize the mother, but stigmatize the illegitimate child.
So, if you think about it, NT writers had to overcome a disincentive to report it at all, since the very mention of it would play into the hands of their enemies. They only record it because that's what happened, even though it hands enemies of the faith a propaganda coup. Sometimes you have to tell a true story knowing that people will twist the truth.
4. Now, a critic might object that my explanation misses the point. Given the rumors of a prenuptial scandal, they had to say something to squelch the rumors. But there are problems with that objection. For instance:
i) That would be a counterproductive alibi. Rather than draw attention away from the specter of a prenuptial scandal, it would draw attention to the specter of a prenuptial scandal. Hostile readers will view this as a coverup.
ii) If the Gospel writers were attempting to conceal a prenuptial scandal, and if they felt free to invent a cover story, why not just say Jesus was conceived after Mary and Joseph got married? After all, the Incarnation doesn't require a virgin birth. The sinlessness of Jesus doesn't require a virgin birth.
If some people find the story of the virgin birth fishy, there's nothing suspicious about saying he was born to married parents. So that would be a better cover story.
5. But a critic might say that misses the point. If Mary was known to be pregnant out of wedlock, then it's too late for Matthew and Luke to fabricate a cover story that denies that fact. The best they can do is to spray paint it with miraculous whitewash. But there are problems with that objection, even on its own grounds:
i) People who deny the virgin birth typically think Matthew and Luke were written about a century after the birth of Christ. They don't think Matthew or Luke had access to firsthand information about the circumstances surrounding his conception and birth. So what, exactly, is there to rationalize or cover up? By that late date, who knews any better what really happened?
ii) Likewise, even if we take the historicity of Matthew and Luke far more seriously, how many people were really privy to the timing of Mary's pregnancy in relation to her engagement and marriage? Other than some relatives and villagers, who else would know about it? Mary wasn't born famous. She was a nobody. She's one of those people who becomes retroactively famous in association with a famous person. Jesus himself only became relatively famous towards the end of his short life, and even then he was just a local celebrity at the time of his death. Had anyone heard of him outside some pockets in Palestine? So why assume, decades later–when Matthew and Luke were written–that there'd be a widespread rumor about the illegitimacy of Jesus?
iii) Presumably, the target audience for Matthew and Luke are people who don't already know about the life of Christ. So what would possess Matthew and Luke to introduce a cover story about the circumstances of his conception? That would create a problem that hadn't existed before in the mind of the reader. For the average reader would never have reason to suspect anything untoward unless Matthew and Luke gratuitously interject this subterfuge.
Left to their druthers, I wouldn't expect any NT writer to mention the circumstances of Christ's conception if they could avoid it, since the story of the virgin birth will be used against them. It's one of those dilemmas where doing the right thing looks like doing the wrong thing. What's striking, therefore, is that we have even one, much less two Gospels, that record the virgin birth. For they must do that despite the derision which that will provoke.