One of the conflicts in the film centers around Marie and Louis' (Jason Schwartzman) difficulty in producing an heir. In the movie, Louis is afraid of sex. In reality, Louis had phimosis, a condition in which the foreskin of the penis cannot be fully retracted. This was later fixed with an operation, and the couple did in fact conceive.Ah, phimosis. One of those conditions that causes people of sound mind sadly to snicker just a little (hello, priapism). Having read a little history and been in a med school for a few years but never having heard of this, I decided to do a little investigating. However, being the lazy guy that I am, I entrusted my research to the good folks at Wikipedia. I thought the phimosis theory was strange enough, but the Wikipedia posits an even more outlandish theory:
The true cause of the couple's infertility is revealed in a letter written by Marie-Antoinette's brother, Joseph II, to another brother, Leopold II. Joseph in April 1777 visited Louis and Marie-Antoinette in France, and had a frank talk with both of them regarding sexual matters; from this, he discovered that the King slept with his wife for duty rather than pleasure. There was no problem with the King's sexual organs: Joseph wrote, "he has strong perfectly satisfactory erections", and "he sometimes has night-time emissions"; the problem was that when the King and Queen slept together, "he introduces the member, stays there without moving for about two minutes, withdraws without ejaculating but still erect, and bids goodnight...when he is inside and going at it...[ejaculation] never happens." In the Emperor's opinion, the pair were "two complete blunderers", who had nothing wrong with them aside from lack of sexual knowledge and desire (Lassonne had already opined in 1773 that the lack of consummation was down to "clumsiness and ignorance").
Joseph, it would appear, remedied the couple's ignorance during his 'talks' with the pair; by August, the marriage was finally consummated, and the pair had thanked him for his advice, to which they attributed the consummation.