I'm all for scientists shining a light on the murkier recesses of our sex lives. It is the mark of a progressive, open, grown-up society as opposed to one that sniggers when anyone says the word clitoris. Without them, we'd all still think babies came from storks. Or as the psychologist JohnB Watson put it early last century: “[Sex] is the thing that causes the most shipwrecks in the happiness of men and women. We should have our questions answered not by our mothers and grandmothers, not by priests in the interests of middle-class mores...[but by] students of sex.”

The problem comes when someone shines a light on the scientists. In all their Petri-dish detail. Particularly when that someone is doing it for laughs. In Mary Roach's rambling tour of the history of science and sex, we lurch from one gratuitous, random anecdote to the next. Chapter one, logically enough, is on the pioneers of sexual response (though my memory of it is obliterated by the revelation in a footnote that Alfred Kinsey put a toothbrush in his urethra). Chapter two segues wildly into sex machines, before Roach settles gleefully into intercourse orgasm, man-assisted pig sex and a whole passage on Mr and Mrs Roach copulating in an MRI scanner (lots of japes, not much on what the scan actually shows).

By the time we move from women's bits to men's bits, you'll be as confused as an ancient Greek sex therapist. And, my goodness, they were confused. You must then battle on bravely through Taiwanese impotence-curing surgery, the pros and mainly cons of attaching additional testicles, and a thoroughly upsetting chapter entitled Re-Member Me (I don't need to tell you what that's about, do I?). Like the second half of a bad rugby match, the structure then breaks down completely, with a discourse on whether a clitoris is a tiny penis, then on masturbation, then on chimpanzees.

One thing that books about sex, even light-hearted, fluffy ones like this, should not be is confusing. It's hard enough not being paranoid about one's bedroom gymnastics without having a lot of frightening, dispiriting half-facts popping into your head as you go. Was it gibbons or women that prefer to be on top? Is sex à la vache good or bad? Can it really be true that the San Francisco fire department has a specially converted circular saw for removing stubborn penis rings? Ignore the image of a woman with a prolapsed uterus orgasming at the drop of a hat. Ditto the man who climaxed when he excreted. Double ditto the man who did the same thing, only the other way round. Roach also reports on a mathematician unable to perform his husbandly duties because each time he tried, a complicated mathematical puzzle distracted him.

Repeatedly, potentially revelatory subject matter is obscured and confused by the author's sniggering approach. I can be as sniggery as the next overgrown schoolboy when it comes to talk of rectal probes and clamping vaginas - I don't need the narrator to do the snickering for me. Yet although Roach expresses the hope in the foreword (predictably entitled Foreplay) that she will not make the reader cringe, she does the exact opposite. Repeatedly and with clear relish. I won't recover soon from the likening of vaginas to Crate & Barrel, of vaginal fistulas to the Three Tenors or of a penis to a boomerang (except an uprooted penis won't come back if you throw it... “it will most likely, and who can blame it, want nothing to do with you”).

If you can make it all the way through the chapter on how Danish men stimulate pigs, you are braver and stranger than I. (And yes, you're right, the big question is not how, but why. And the big answer is not because they can, but because pigs make more piglets when they enjoy the conception. Roach, though, always lingers on the how.)

You may be fine, but when I reached the “What's Going on in There” section on coital imaging, it was too early in the morning, so I skipped it, only to find my wife enjoying it. “I never knew that...I never knew that...I never knew that,” she said disconcertingly before giving up after one too many dildo jokes.

Roach is clearly dedicated (putting herself and her husband through several experiments) and her book is well researched (my God, those footnotes), but it would have been much less stressful if she'd just e-mailed me the nuggets of genuinely fascinating sex-science trivia and spared me the gory sideshow detail. Still, I now know that the way to tell if a sow is on heat is if she lets you sit on her; that tall, big-breasted women are harder to please in bed; that the normal range for a flaccid penis is from 1.6 to 4.7 inches (phew); and that it is technically possible for corpses to orgasm. The fact that my sex life is in tatters seems a small price to pay.

Bonk: The Curious Coupling of Sex and Science by Mary Roach
Canongate £12.99 pp319 Buy from Books First £11.69 including delivery Buy the book

Times Recommends

• When You Are Engulfed in Flames by David Sedaris
• The Spare Room by Helen Garner
• Is Bertie Wooster the only fish-faced hero?