Proving a woman has living descendants means precisely that said woman, at one point, had, at least, one child and that her child(ren) also reproduced. There is absolutely no way to prove paternity through maternity testing. If Mary Magdalene had any children, that just makes her one of the many saints, both male and female, who have done so. It doesn't tell you whether she was married, or to whom and without a male to test for paternity, it can't tell whether the father is Jesus or some random Tom, Dick, or Harry from down the street.
In fact, you can't even say that the bones in the Priory's possession are those of Mary Magdalene. The most you can say is that they belonged to woman of Hebrew descent who lived approximately 2000 years ago.
Which would be why the Priory preserved an enormous amount of historical records corroborating her identity.
Still, a woman can claim any man is the father of her child(ren). Other people can claim that the man is the father of her child(ren). Unless the man's DNA is tested against that of the child(ren) and paternity is established, someone saying that the man isn't the father is just as valid. Many saints have borne or fathered children; Mary Magdalene would be no different, and in this day and age, the Catholic Church wouldn't need some giant conspiracy. Maternity testing would establish Mary Magdalene's bloodline, but with no known remains of Jesus to prove or disprove, it wouldn't matter if people believed that it was also his. They couldn't prove he was the father of the bloodline, and the Catholic Church couldn't prove he wasn't.
The Priory of Scion is a society that celebrates the sexual union between man and woman, and includes rituals involving heterosexual intercourse. But the book says one of their leaders was Leonardo Da Vinci. Who was quite well known to be gay.
Well, maybe. No heterosexual or homosexual relationships have been confirmed, and the sodomy charges against him were dropped due to lack of evidence.
Clearly anyone who thinks this book is about unraveling and Ancient Conspiracy, is offensive to Christians, or finding the Holy Grail has not read the ending. The Conspiracy Theorist who presents the entire background of the quest turns out to be the villain behind the whole plot, the conservative Bishop ends up just being out to save his Church because his followers have a bad reputation thanks to the media, Silas was far removed from the norm of Opus Dei, but even he is wracked with guilt and regret, and the Priory is Sion is shown to be all but purely symbolic. They never even intended to "reveal the secrets" to the world because that is not the purpose of the secrets. Langdon ends up following the final clue back to where the story began, but instead of trying to uncover the bones of Mary Magdalene and show the truth to the world (which isn't even confirmed to be true), he kneels before the symbol of the sacred feminine and pays his respects to what it represents. The purpose of the quest was always to find the truth, but in a spiritual or metaphysical sense. To this Troper, the ultimate purpose of this novel is not be be correct (the author is the trope namer for Dan Browning after all), but to encourage the reader to think and open their mind to new possibilities without outright rejecting the old ones. It is an Author Tract on spirituality masquerading as a conspiracy thriller.
If the Holy Chalice (aka the Holy Grail) is an allegory for Mary Magdalene, and the Gospels of Matthew, Mark and Luke describe him passing the Holy Chalice around during the last supper, that means he was actually passing his wife around to his apostles. So basically, The Da Vinci Code tells the story of how Jesus was a swinger.