When it was announced that conjoined twins Chang and Eng Bunker, best known as The Siamese Twins, were planning to come to France in 1831, French authorities were so afraid of the effect the men, then 20, would have on France’s women that they banned their entrance into the country.

While the concept of conjoined twins — two independent people permanently joined as one — is intriguing for many reasons, few aspects spur as much curiosity as how two such people live romantic, sexual lives.

According to author Joseph Andrew Orser’s new book, “The Lives of Chang & Eng,” the Bunkers, born in Siam in 1811 and connected at the midsection by a fleshy band several inches long, were spotted in their teens by a British merchant who first thought they were “some strange animal.” When they turned 18, he made a deal to bring them to America and exhibit them as public curiosities.

Upon their arrival, they were subject to countless medical inquiries. One doctor, testing their connecting band with needles to determine sensitivity, found that “both boys drew away from punctures at the middle of the band, whereas at half an inch or more from the center, only the twin on that side felt the pain.”

He also found that “when one experienced a sour taste, the other did as well,” and that “tickling one of them resulted in the other demanding a stop to it.”

More than their connective similarities, though, the public wondered about the boys’ potential sex lives. One story held that Chang interfered in one of Eng’s pursuits, and that, according to one newspaper, “the brothers would have engaged in a duel, but ‘the parties could not agree on a distance.’ ” This and other tales were more than likely unfounded, but provided opportunities for public mockery.

“The prospect of the twins engaging in sexual relations with women disturbed sensibilities,” Orser writes. “Concerns existed about the impact that the twins conjoinedness might have on women of childbearing age.”

In one extreme example, when a woman in Kentucky gave birth to stillborn conjoined twins, she “claimed she had seen numerous representations of the twins in newspaper advertisements around the time she conceived her children, which affected her imagination.”

The brothers gained fame as freaks, and saw opportunity as Americans. After a decade on the sideshow circuit, having saved some money, they retired, bought land in North Carolina, and set out to create lives for themselves as proper Southern gentlemen. They bought property, became US citizens, and even took on slaves — ironic, considering that throughout their early lives here, many questioned whether, despite their firm denials, they were slaves themselves.

In 1843, Chang and Eng married, respectively, sisters Adelaide and Sarah Yates, daughters of a respected local landowner. While the girls had a “fair share of suitors,” the brothers had gotten to know them over several years, often visiting upon their return from business travels, and befriending the entire family.

When the couples “made their intentions to marry known by riding together in an open wagon,” one report of the time cites how “all hell broke loose.” “A few men ‘smashed through some windows at [the girls’ father’s] farm house,’” and some of his neighbors “threatened to burn his crops if he did not promise to control his daughters.”

The local media reacted to the unions with jibes. The Carolina Watchman, in a post titled “Marriage Extraordinaire,” wished for the marriage to be “as happy as it will be close.” Another paper inquired as to whether the women ought to be indicted for “marrying a quadruped.”

Northern newspapers were appalled, as abolitionist papers placed “responsibility for the union squarely on a South contaminated by the sin of slavery.” One paper even called the marriage “bestial,” and referred to the tolerant local residents as “a community sunk below the very Sodomites in lasciviousness.”

For their part, though, the two couples — and they were, unquestionably, two distinct couples, coming to live in separate homes, with the brothers alternating half weeks in each — sought little more than normal lives.

But many among the public and the media, having barely brought themselves to tolerate the brothers’ existence, found the concept of intimate relations between them and “normal” women a step too far.

Each wife gave birth in 1844. While no details survived about how the couples conducted their intimacy, it’s worth noting that the brothers’ first children were born six days apart, and a later pair eight days. (They would go on to have an astounding 21 children between them.)

When the twins, in need of money, later returned to touring exhibitions, this time bringing two of their children along, many refused to accept this unconventional family.

As they traveled through England, some in the British press “doubted whether the ‘family’ was even real,” Orser writes. “For some, it was too ‘disgusting’ to imagine these ‘human monsters’ as husbands or fathers.”

In 1870, Chang suffered a stroke that “paralyzed his right side, the side closest to his brother.” Eng nursed him back to relative health as Chang “tied up his right leg in a sling” and, using both a crutch and his brother’s arm, went about his daily routine.

But he never returned to full health, and took to drinking. A lingering cough later turned vicious, and he died on Jan. 17, 1874. His brother, complaining of ill health, asked his son to check on his brother. Told that Chang had passed, Eng replied, “Then I am going.”

Over the next hour, he “suffered intense pain and distress, a cold sweat covering his body. The only notice he took of his dead twin was to move his body nearer to him.”

Two-and-a-half hours after losing his brother, Eng Bunker died.