Certainly in the case of Marie Antoinette, this is inaccurate. In fact, her marriage to Louis XVI was notable, in its first few years, for the couple's lack of children. Married at 14, to the 15-year-old Dauphin, her first pregnancy did not occur until 8 years later. Historians have speculated as to the reason for this - it can't have been infertility, as the couple went on to have a further three children, and the theory that Louis suffered from phimosis has been discredited by the fact that no circumcision surgery (the only solution at this time) has been recorded. All the evidence points towards lack of sexual knowledge on the part of both husband and wife, as evidenced by letters from Marie Antoinette's brother, the Holy Roman Emperor Joseph I, who visited them incognito in 1777. Writing to his brother Leopold back in Vienna, he relates having frank conversations with both of them, separately, on the topic of intercourse, and refers to them as 'complete blunderers' in bed. (See Marie Antoinette by Antonia Fraser; also Louis and Antoinette by Vincent Cronin).

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It is highly unlikely that elite unmarried women in the 18th century were ever deliberately 'introduced' to sex in this way. While pornographic engravings and pamphlets were available - Aretino's Postures are probably the best known - they were aimed largely at an upper- and middle-class male audience. Virginity and modesty were prized characteristics in a potential wife, given the relationship between legitimate childbearing and the transmission of heritable property and titles. It was theorised that exposure to sex made women lustful - see, for example, the many descriptions of sex workers in Harris' List of Covent Garden Ladies (published annually between 1757-95), which frequently characterised a woman's participation in prostitution as being the result of sexual appetites being raised by 'seduction' (which could mean consensual pre-marital sex, or rape).

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Edit: typo

Since no one has taken a crack at it yet, allow me to throw my hat in the ring. I'd like to re purpose an answer I made to a previous question here, but with appropriate edits. I'd like to focus on Qing Dynasty Emperors during the latter years of the Empire.

The idea of self-restraint was one of the philosophies that were greatly valued in the Qing Court. Emperors aspired (or at least, claimed to aspire) to be the ideal Confucian monarch, and a good Confucian monarch should be a paragon of self-restraint. Such self-restraint even extended to sex, which led to the formulation of ideals and lessons on sex:

For my source I am primarily using Sterling Seagrave's Dragon Lady: The Life and Legend of the Last Empress of China, which chronicles the life of Empress Dowager Cixi, starting from her time as a lowly concubine to (arguably) the most powerful figure in the court during her time.

In one passage, Seagrave recounts the Taoist dogma about sex, which advises and educates the emperor on what to do:

According to Taoist theory, emperors needed as many wives and concubines as possible to maintain a balance between the life forces of yang and yin, to acquire the potency needed to father a Son of Heaven. Woman's yin essence, her fluids, were thought to be inexhaustible, while man's yang essence, his semen, was limited. To build up the emperor's yang required a great deal of yin. The best way for the female life force to transer to the emperor was for him to engage in a lot of sex with his concubines without ejaculating (emphasis mine)

Sounds tough? That was not all the emperor was expected to do:

While restraining himself, he must rouse his concubine to repeated orgasms. Taoists boasted that this exceptional self-restraint produced supernatural powers ... The more women partners an emperor had, they said, and the more often, the better, so as he restrained himself with every single one. A Chinese sex manual blandly asserts: "If in one night he can have intercourse with more than ten women it is best." Sticking to the same woman was dangerous, because her vital essence would grow weaker, until she was no use at all.

Tips were also provided to avoid orgasms, such as:

At the last moment, the man closes his eyes and concentrated his thoughts, he presses his tongue against the roof of his mouth, bend his back, and stretches his neck. He opens his nostrils wide and squares his shoulders, closes his mouth, and sucks in his breath.

To help with such a monumental task, sex toys were even brought into play, such as the innocuously nameds Sheep's Eyelid:

This curious object, kept in a tiny jade box by the emperor's bed, was a circle of finely tanned skin cut from around a sheep's eye, including upper and lower lids, eyelashes intact. After immersion in a cup of hot tea, it was slipped on like a soft ring, so that the eyelashes fanned out. With its help as a tickler, there was little difficulty generating all the ying an emperor could use from his concubines.

So what was an emperor to do with all this pent-up raging semen?

After building himself up over a month with concubines, the emperor was to couple with his empress just once, when the best chance existed for her to conceive an heir to the throne.

Of course, this practice wasn't always observed by the Manchu emperors - and even if they did, it didn't always work, as none of the three four (Hsien Feng, Tung Chi, Guang Hsu, and Pu Yi) last emperors of the Manchu dynasty produced heirs with the Empress Consorts. But that is a discussion for another time.

But of course, despite the exhaustive enumeration of odd sexual practices, it still doesn't answer the question of how the Emperors were taught about sex.

The answer could be through a variety of people, and even through pornography.

First we have the Imperial Doctors. The idea of yang essence dangerously seeping out of the emperor because of not properly following the ideal rules were considered a plausible diagnosis for ill health back then. In fact, physicians specified how many ejaculations a man could dare have without damaging his system: once every three days in spring, twice a month in summer and autumn, and not at all during the winter. One accidental orgasm in the winter was a hundred times more exhausting that one in spring.

There was also the existence of record keepers (eunuchs or court ladies, depending on the period), who supervised an emperor's sexual contacts. Their supervision mostly had to do with making sure that the ruler coupled with his various wives on the correct days, according to the calendars, and the frequency established by the rites for each rank of concubine. In a way - they were set with the task of educating the emperor on How To Have Sex, Chinese Emperor Style.

But did that mean that Chinese Emperors/Princes did not do any ehem... research of their own?

Of course they did. It was no uncommon for Princes who have reached puberty to venture out into brothels - some of them even took their pleasure with their own eunuchs. In fact, this was exactly the case with Emperor Tongzhi, biological son of Empress Dowager Cixi.

He had ascended the throne as a child and by the time he had reached fourteen, he was already the subject of a growing scandal regarding his behavior. The young Emperor was known to slip out of the Forbidden City and visit theaters, brothels, et al. where he took his pleasure among prostitutes.

If pornography was needed to (for education purposes), then one had to look no further than the storerooms of the Forbidden City. Decades ago, Emperor Chien Lung, like others before him, had commissioned many pornographic works of art, which were available from the storerooms of the Forbidden City. The collection included The Art of the Bedchamber, Important Guidelines to the Jade Room, and Secret Dissections of Health Maintenance. The last title included advice on how to increase the size of a man's genitals!

Robert Hart, who was a long-time resident of Peking and an eventual officer of the Qing Court, even writes that Emperor Tung Chih was provided early on with a very young girl to initiate him into heterosexual pursuits, in an attempt to draw him away from his homosexual fixation. Unfortunately, Hart does not write who was instrumental in procuring the girl, so at best we can only speculate. However, safe to say that demonstrations were not off the table when it came to educating the monarch.

I hope this contributes, even in part!