Brian stands 6ft tall with a dash of Matt Damon in his dentally flawless smile. Little wonder the Ford model agency offered him a contract. Big wonder that at 24 he remains a virgin. In fact, Brian has never been past first base (hand holding/kissing), hasn’t fondled a woman, let alone gotten naked. Indeed he tries to conduct dates in public places to ensure he isn’t led into temptation, so that he can keep his pledge to God that the first time he has sex will be on his wedding night with his wife.

I should add that I didn’t find Brian through a brimstone ministry. He was just the friendly guy who served me a scoop of cherry-vanilla at a coffee shop in the small Ohio town where I’ve spent the week, and we got talking. About temptation (“Sister, it can be hard”), masturbation (best avoided because it just multiplies lustful thoughts) and the big night itself (“I guess it could be an anti-climax”).

He is a sweet guy, from a large non-churchy family of military and medics, who works the 6.30am shift to pay his way through nursing college. I’d wager he isn’t unusually screwed up or repressing homosexual desires. Neither is he a humourless funsucker – he likes indie rock and a beer or three and mixes with atheist and gay friends.

But as I look into Brian’s guileless green eyes I recall Russell Brand’s ill-received jokes at the MTV Video awards. Speaking of the Jonas Brothers, a squeaky-clean American boyband outfit who wear “purity rings” to avow their commitment to remain virgins until marriage, Brand, the hoary old sex addict, was bemused. “But they could get loads of girls. It’s like Superman deciding not to use his special power to fly and just taking the bus.”

And I spend my conversation with Brian repressing a bubbling snigger. Why, I want to cry out, are you wasting your prime shagging years? (His big brother, a Marine, says much the same thing.) What if you end up like the local youth pastor I just spoke to, still waiting – at 36! – to find a woman who wants to receive his “greatest gift of love”?

Having lived only in Los Angeles, Brand made the classic British error of underestimating Christianity’s hold over Middle American life. This week, I feel suddenly very far from home: meeting high-school teachers and businesswomen – educated, affluent folk, not ignorant rednecks – who think teaching creationism on an equal footing to evolution is “fantastic because it will allow children to choose”, or seeing a gaggle of moms who look just like my London mates, planning a school trip with reference to the book What Would Jesus Do? America is the Moon landing and Microsoft, the apogee of the modern, and yet sometimes you think the Enlightenment never happened.

From Europe it is easy to assume that this virulent religiosity brings about only bigotry and unreason. What I didn’t appreciate until this my first visit to the mid-West is how belief can translate into such unrivalled courtesy and generosity, my days here lit up by hourly acts of kindness.

In our snarky, mickey-taking, Brandish superiority we decline to understand why so many American teenagers might become denim-clad nuns. We might note that billions of dollars spent under George Bush on abstinence-only sex education has done nothing to reduce a teen-pregnancy rate more shameful even than our own – and which sure didn’t work for Bristol Palin.

Stars such as Britney Spears have fuelled our cynicism by making a virginity pledge seem just a convenient branding to give a look-don’t-touch respectability to under-age raunch. Teen star Miley Cyrus wears her own purity ring – purchased at Disney World – while looking lipstick-smudged and postcoital for Vanity Fair. Virginity is hot; avowed chastity even hotter. All that repressed desire, the lip-smacking possibility of seduction, touching someone, as Madonna understood so well, for the very first time. Perhaps the Jonas Brothers are sincere but just maybe it sells more CDs if a tweenie girl fan can believe that Kevin or Joe or Nick is saving himself just for her.

And the craze for purity rings has a cheap whiff of fashion, providing yet another reason to hit the mall, this time for a $200 gold band inscribed with “True Love Waits” or bearing an unblossomed rose. One mother told me her 11-year-old daughter wears one – as if a primary school child needs to assert her virginity or can even identify sexual feelings – but her other daughter abruptly lost hers at 18 once she found a boyfriend. A father tells me how the conditions of the pledge are often interpreted in Clintonesque style, with oral or even anal intercourse not counting as a breach of rules.

But those like Brian – who sees no need for a ring to advertise his beliefs – aren’t just obeying the thump of a pulpit or I Thessalonians iv, 3-4. The spread of the evangelical movement is partly why young Americans are often more socially conservative than their own Sixties-raised parents. They are also reacting against a popular culture that commodifies sex, pimps the young and is seeped in pornography that twists their burgeoning desires into sordid, loveless XXX-rated couplings. A father tells me of high-school party races in which girls performing fellatio on boys they might not even know compete to finish first.

And thus Brian mirrors the reasons I’ve heard British Muslim girls give for wearing the veil: to remove themselves from a sexed-up society they don’t respect because it does not respect them. These girls see a clear choice between the hijab and the thong. And this same moral absolutism permeates the purity movement. Can we shun porn culture only by regressing to a time when masturbation is shameful? Is it necessary for white-clad teenage virgins to attend America’s popular father-daughter purity balls where dad pledges to protect his child’s chastity until she can bestow it upon her eventual husband?

Isn’t there some middle course between STD-infested sluttiness and this perverse repression of God-given appetites? I fear Brian may be inflating the sweaty, animal business of copulation into some sacred rite. It is, after all, only sex. But he just smiles his easy Midwest smile and says he has a girl in mind. She’s saving herself too. Once he’s qualiied as a nurse, they’ll wed. Not too long now then? “Sister,” he says with feeling, “I can’t wait.”