What point-of-view are these two women coming from? What do they mean by "fabricated," exactly? The virgin birth is in the Gospels, which are substantially the same in all manuscripts, apart from some early manuscripts of Mark without the Resurrection appearances. If your interlocutors are hostile to traditional Christian beliefs, it is probably a fruitless conversation to have.
Let me show what they write below. Let me know your thoughts and how you would counter it. One of them has quoted a Christian scholar.
Scholar Affirms That Later Editor Deliberately Corrupted Text to Add Virgin Birth This is agreed upon by J.R. Wilkinson - a renown Christian scholar of his day -- in his article "Mr. Conybeare's Textual Theories, Hibbert Journal, October 1902, p. 96," Hibbert' Journal (1904) at 354 et seq. We read at page 358 (link) the following text: The reading of Syrus Sinaiticus Joseph to whom was betrothed Mary the Virgin begat Jesus who was called the Christ is clearly due to a mixture of the Western text with the reading of the original genealogy. [Note: the Western text has the virgin birth.] It carries us back to the early days when the canonical gospel was competing with the source or sources from which it was derived.
We have here perhaps a conscious corruption of the canonical text for we find that in Syriac Sinaiticus agrees with the famous Old Latin manuscript in omitting the words mix é y vwo xev a1i ri1v Ewg [MY NOTE: verse 25, 'before they came together, found with child" omitted in Old Latin and Syriac Sinaiticus thus proving the Older manuscripts did not have virgin birth.] The reading of Syriac Sinaiticus [of 1:25] here is he took his wife and she bore to him a son and he called his name Jesus. The reading of Timothy and Aquila is as Schmiedel says a confluence from the Textus Receptus [i.e., Virgin Birth] and the original genealogy [i.e., "Joseph begat Jesus."] The article continues and weighs the possibility that the "canonical evangelist" (not Matthew) may have not been sure which way to go, and left traces of the original version -- Joseph begat Mary, while trying to conform to the newer virgin birth account. He calls the "Davidic Messiahship" idea was the "old Davidic idea" which was gradually being displaced by the newer "supernatural birth idea" in the mind of the "canonical evangelist" himself. The article ends suggesting that the "canonical evangelist" was not the original compiler of the sayings in Matthew, and that he used a version of Matthew "much in favor in the church" which had the older text, and then would "corrupt by mixture the old text," i.e., mixing mention of the "virgin Mary" in 1:16 with "Joseph begat Jesus." So now it reads in Syr Sinaiticus 1:16 partly in favor of a virgin birth and partly negating it. The scholar Wilkinson earlier had agreed on why we are compelled to believe both Matthew and Luke had to have something like "Joseph begat Jesus." Otherwise, the point of the geneologies is lost: [355] The text of Syrus Sinaiticus alone seems to postulate such a clause [i.e., Joseph begat Jesus]; and is it possible to believe anyone would have taken the trouble to construct the long line of natural descent if at the last step natural descent were to play no part? This inconsistency is best seen in the geneaology of St. Luke, where as Holtzmann [Ham-Kommentatur Zum N.T.] justly remarks, it is absurd to assume an evangelist would take the trouble to construct the long geneaology of our Lord through Joseph, and then as it were, spoil all that he had done by adding that Jesus was only "accounted" or supposed to the son of Joseph. The construction of such genealogies presupposes natural descent throughout. It is evident that in both St. Matthew and Luke correction has taken place [356] (in the case of St. Luke a very clumsy correction), so as to render these genealogies consistent with a virgin birth.
and
this:
nvalidates Jesus As Messiah The first problem with the virgin birth account is that it makes it impossible for Jesus to be Messiah. He must come from David's flesh and blood. Jewish critics correctly say: The Messiah must be descended on his father’s side from King David (see Genesis 49:10 and Isaiah 11:1). According to the Christian claim that Jesus was the product of a virgin birth, he had no father— and thus could not have possibly fulfilled the messianic requirement of being descended on his father’s side from King David! (Rabbi Simmons, “Jesus as the Messiah,”
http://judaism.about.com/library/3_askrabbi_o/bl_simmons_messiah3.htm (August 14, 2011). See also About.Com Judaism, "The Jewish View of Jesus.") Jerome knew an adoption by Joseph does not satisfy the prophecies in Samuel, Isaiah and Jeremiah that Messiah must be in the blood line of David. So he made up a claim that it is IMPLIED that Mary is of the Davidic line -- a completely unfounded claim. Yet, this proves that Jerome knew something is missing -- fatally -- due to the virgin birth account. First, Wikipedia mentions: “The Catholic Encyclopedia (1913) states [there is]... a general implication of her Davidic origin.....’ (“Genealogy of Jesus,” Wikipedia.) Finally, Jerome's words from the 380s in his Commentary on Matthew 1:18 say: “Since Joseph is not the father of the Lord and Savior, how does the sequence of the genealogy descending to Joseph pertain to the Lord? Our first response to this is that it is not the custom of Scripture to trace out genealogies of women; secondly, Mary and Joseph came from the same tribe. This is why he was compelled by law to marry her, since she was a close relative. Also the fact they are both counted together [in the census] at Bethlehem clearly shows they were descended from the same stock.” This is utterly bogus. The accounts of the lineage in both Matthew and Luke are of Joseph's line, not Mary's. They are said to be taken in a census in Bethlehem because Joseph (not Mary) is in the Davidic line. In Luke 2:4, Luke says that Joseph in going to Judaea went “unto the city of David, which is called Bethlehem, because he was of the house and lineage of David.” But Jerome's bogus claim proves one thing: even Jerome knew an adoption by Joseph does not fulfill the prophecies of a Davidic Messiah. He and the Catholic Church desperately seek the answer in the nonsensical notion that Mary was supposedly in the Davidic line. Hence, because the virgin birth makes it impossible to believe Jesus was of the Davidic bloodline, we must examine the evidence to the contrary which lifts this burden off of Jesus' shoulders.. Certainly, adoption by Joseph is clearly not what the prophecy in Samuel had in mind. Thus, the virgin birth story is fatal to Jesus' validity as Messiah, thus dictating we must examine the compelling evidence that the virgin birth account was not originally present.