Why isn't the virgin birth of Jesus mentioned in all of the gospels? Does it matter? I recently saw Bill Maher's documentary and he mentions that the virgin birth is only mentioned in two of the gospels. He goes on to bug Christian people several times by asking questions such as "How can you believe in the virgin birth if it's only mentioned in two of the gospels?". I know for a fact that the virgin birth is mentioned in the gospels of Matthew (1:18) and Luke (1:26-35), however, it is mentioned in neither Mark nor John. In spite of that, millions of Christians world-wide take the virgin birth for granted. My question is, as stated above, why isn't the virgin birth of Jesus mentioned in all four gospels?
The birth of Jesus is also only mentioned at all in Matthew and Luke.  Following the logic, since only half of the Gospels mention that means we should discard the theory that he was ever born at all. 

In 100% of the cases where a birth narrative is present, the miracle of God becoming man is mentioned. Indeed, in John, even where the birth narrative isn't present, the incarnation - that God became man - is explicit. 

Neither John nor Mark say anything about the birth of Jesus at all, so it is not surprising at all that there is nothing about the virgin birth.