One thing we know for sure about the sexuality of conjoined twins: People who aren’t conjoined are fascinated by it. At least it seems that way, judging by the number of reporters calling me to ask about the sex lives of conjoined twins since the TLC reality show Abby and Brittany went on the air several weeks ago. As I’ve told callers, although there are no real studies of the sex lives of conjoined twins, we can safely assume that conjoined twins want—and occasionally feel conflicted about wanting—sex, as we all do.

But not as conflicted as we singletons seem to feel about them having sex. Typically, people who are close to conjoined twins come to adjust and see them as different but normal; they seem fairly untroubled by the idea of conjoined twins pursuing sex and romance. But those who are watching from afar cannot abide.

The best example of this would probably be the story of Chang and Eng Bunker, the “Siamese Twins,” so called because they were from Siam (now Thailand). Chang and Eng were joined by just a bit of liver and some skin. One April day in 1843, Chang married Adelaide Yates, while brother Eng married sister Sallie Yates. Based on the fact that Chang and Adelaide had 10 children, and Eng and Sallie, 12, it’s fair to say the brothers had sex.

At the autopsy of the Bunker twins, one of the anatomists opined that their active sex lives had “shocked the moral sense of the community”—even though the truth is that the Bunkers’ neighbors appeared to have just accepted the situation. A little-known fact is that the Bunker wives’ father had originally objected to his daughters marrying the twins not because they were conjoined, but because they were Asian. (This was, after all, the antebellum American South.)

Yet in the 19th century, when doctors discussed whether the twins Millie and Christina McCoy could marry, one spoke for many: “Physically there are no serious objections ... but morally there was a most decided one.” When, in the 1930s, Violet Hilton sought to get a marriage license while conjoined to her sister Daisy, she was repeatedly refused.

The same discomfort generally carries through to our own time. When the filmmaker Ellen Weissbrod set out to do an A&E program about Lori and Reba Schappell, who are conjoined at the face, Weissbrod showed raw footage of the twins to New Yorkers on the street, without explanation, to gauge their reactions. Upon figuring out what she was looking at, one woman said only, “I mean ... sex ...”

Sex is often mentioned by commentators on conjoinment as one of the beautiful things supposedly made instantaneously horrible by being conjoined. I’m afraid I just laughed when, in writing a book on conjoined twins, I came across this 1984 line by a nurse writing in a medical journal: “Two people never being able to obtain privacy to bathe, excrete, copulate, or eat defies imagination.”

Surgeons sometimes openly allude to sexuality as a motivator for separation surgery. In 2002, as soon as he had made the cut separating two little girls joined at the head, the neurosurgeon involved paused to announce to the assembled medical team, “We now have two weddings to go to.” Indeed, when I talked to contemporary surgeons about how they decide whether to undertake the substantial risks some separations involve, I found that surgeons had two fears, sort of conjoined: one, that twins would grow up conjoined and thus never have sex; two, that twins would grow up conjoined and actually have sex.

Believe it or not, surgeons have done this: Separated toddler twin boys and made one a girl because there was only one penis to go around. (These children were essentially two people on top and one on the bottom.) In fact, this has been done in two cases. In one case, the “girl” is said to have reverted to being a boy, and in the other, the child left as male died, leaving the parents who had come to the hospital with two sons to go home with one daughter. Yes, this was considered better than leaving the children alone.

So I suppose I should get to what the people really want to know: What do conjoined twins feel when they have sex? If one is sexually stimulated, does the other feel it? If one has an orgasm, does the other enjoy the same, however unwittingly?

The short answer is that we don’t know. Conjoined twins, like the rest of us, tend not to talk in great depth publicly about their most-intimate moments. Based on what we know about the significant variability of one conjoined twin’s feeling a body part (e.g., an arm) that putatively “belongs” to the other twin, it’s hard to guess how any conjoinment will turn out in practice. Nerves, muscles, hormones, and psychology all probably factor into who feels what. If twins share one set of genitals, they’re both going to feel any touching down there. Whether or not both are "having sex" with the third person in the equation depends on how you think about “having sex.”

One reporter calling about the TLC reality show asked me, “If Abby Hensel is kissed, will her sister Brittany feel it?” The biology geek in me wants to answer that the happy hormones that come from a good kiss probably work their way to both brains. But the student of human nature in me says that when your sister gets kissed and you don’t, it’s quite possible that the unhappy hormones end up standing at the gate.

From my studies, I would postulate that conjoined twins probably end up having less sex than average people, and that is not only because sex partners are harder to find when you’re conjoined. Conjoined twins simply may not need sex-romance partners as much as the rest of us do. Throughout time and space, they have described their condition as something like being attached to a soul mate. They may just not desperately need a third, just as most of us with a second to whom we are very attached don’t need a third—even when the sex gets old.

But when a conjoined twin has sex with a third person, is the sex—by virtue of the conjoinment—incestuous? Homosexual? Group sex? Well, it definitely is sex. You can tell because everyone wants to talk about it.