About the myth of Louis' "impotence:" there is no proof whatsoever that he ever had that problem. Louis and Antoinette, married at ages 15 and 14, took awhile to consummate their marriage. This was not uncommon - Mme de Lamballe's huband died when she was 17 and their marriage had never been consummated, so she claimed. It had become fashionable for some couples to wait until they got to know each other better. Yes, I know the families were having fits over it, but Louis had a mind of his own.
Bernard Fay, Louis' biographer, quotes the Spanish Ambassador, who paid spies to check the sheets of the young couple, claiming that there was evidence of sexual activity early on in the marriage, but Marie-Antoinette may have had a physical problem that hindered the full consummation of the union. The "little operation" she mentioned to her mother in a letter, may have meant procedures other than the circumcision allegedly performed on Louis.
Vincent Cronin, in "Louis and Antoinette," says that during the time that Louis had the alleged operation to get rid of the phimosis (circumcision), he went horseback riding everyday, which would have been physically impossible. Also, Louis' medical records (every prince received a thorough medical examination when they left the nursery) make no mention of a phimosis. Louis may not have had any physical problem at all.
Marie-Antoinette's brother Joseph made his incognito visit in 1777 to see why they had not produced an heir, and he wrote a graphic letter to Leopold which Vincent Cronin thinks was to have a laugh at Louis' expense, blaming all the problems on him. (After all, who had ever heard ofa Habsburg with infertility problems?) The main problem with Louis and Antoinette, is that he went to bed early and she stayed up late. Once they got their schedules coordinated, they conceived.