For a good 10 minutes Mr Kim, a committee official, had been enthusiastically reciting the strategic advantages of Rajin's harbour. Perhaps it was my less-than-rapt expression that caused him to make a drastic change of subject.
"I understand," he suddenly said, in slow, formal English, "that there are many concubines in your country." I admitted that there probably were. "Would you be so kind as to tell me how much a concubine costs?" he asked. I guessed around pounds 40. "So is that for the night," Mr Kim wanted to know, "or just for f---ing?"
North Korea does not prepare you for exchanges like that. The country has been virtually closed to foreigners since the Korean War and, like the cities of Sonbong and Rajin, our hosts at the investment forum had been carefully primed for our arrival. But conversations such as the one with Mr Kim persuaded me that, above harbour depths and container capacities, what North Korean bureaucrats were most interested in was, well, girls.
The standard opener, common throughout Asia, was about marriage, children or, in the absence of these, girlfriends. Frequently, however, questions went beyond the bounds of normal cocktail party conversation. "English women," one man asked, "what size are they?" At the banquet on the first evening of the forum, a foreign businessman was silenced by an even more direct inquiry. "I have a question," he was told by one of the hovering officials. "What is sodomy?"
After his dramatic breaking of the ice, Mr Kim proceeded to fill me in on some of the reasons for this excessive curiosity. Romance, as with most aspects of life in North Korea, is closely controlled. Arranged marriages are still the norm; love marriages do sometimes take place, but are considered unwise, "because there are problems afterwards - too much drinking, and the like".
Sex before marriage is "against the rules, although some people do it anyway". Transgressors are not formally punished, but any who are caught will find their names published, and "the people will criticise them". But we were straying from the original subject. "So," Mr Kim began again, "have you ever been to a whorehouse?"
As it turned out I had, although I did not realise it at the time. The night before, some acquaintances and I were escorted by one of the friendlier guides to the recently opened Rajin International Club. For $30, beyond the means of all but the most privileged North Korean, we enjoyed a hot bath and sauna, watched TV beamed by satellite from Hong Kong and endured a painful massage from a frowning young lady who demanded what seemed an exorbitant tip (our guide was waiting outside). We all declined, excused ourselves from a turn in the karaoke room upstairs, and left.
Conversations with Korean-speaking delegates next day revealed the truth: we had inadvertently wandered into what is possibly North Korea's first brothel.
But prostitution may be one of the few viable industries in Rajin-Sonbong. The area is unappetising for serious foreign investors, having no airport, dismal hotels and a water supply inadequate for anything more than light industry. Its only advantage is its closeness to the booming markets of north-east China, but the Peking government is years ahead of North Korea in its understanding and application of market principles.
At the end of the forum, officials announced more than $280m of foreign investment. Nearly all of this, however, was accounted for by a five-star hotel and casino, aimed by a Hong Kong conglomerate at new-rich Chinese from across the border. Large-scale gambling almost inevitably brings with it other vices, and analysts attending the forum suggested that this may be Rajin-Sonbong's fate. "This place is never going to be a Hong Kong or Singapore," said one. "Maybe their only hope is a kind of Macao, a tax-free gambling and prostitution zone, a gangster's paradise."