In the fading daylight they come out by their dozens -- young men in small groups or alone, cruising Phnom Penh's parks for sex, not with female prostitutes but with each other.
"Having sex with men is just something I like to do, it's relaxing," says a pub manager who gave his name as Pov as he sat among drinkers, all men, at his bar a few blocks off of the capital's busy riverfront.
In Cambodia, as elsewhere in the region, men having sex with men is nothing new.
But Pov's world, like that of many Cambodian men who are known as "MSM," is defined by secrecy and, in some cases, self-delusion and denial.
MSM -- men who have sex with men but who may not consider themselves to be homosexual or bisexual -- account for four percent of Cambodian men, according to experts, and represent a ticking HIV/AIDS timebomb.
Pov admits that his wife in rural Cambodia, whom he sees a couple of times a week, has no idea about his trysts with two or more male partners a month.
But he says he does not consider himself to be homosexual, or even bisexual, despite his predilection for sex with men.
Health workers say this growing and largely unseen trend towards risky sex threatens to seriously undermine progress in tackling one of Asia's worst HIV/AIDS epidemics.
Aggressive condom and sex education campaigns largely targetting Cambodia's sex industry effectively halted a spiralling HIV infection rate that in 1997 peaked at 3.7 percent of the country's approsimately 11.5 million people, making Cambodia the Asia-Pacific region's worst-affected country.
Cambodia's overall HIV prevalence has since dropped to 0.9 percent.
But health officials say they are failing to reach MSM, who have become the most vulnerable to infection.
"There is a very serious concentrated epidemic among MSM," says Tony Lisle, Cambodia's UNAIDS country coordinator.
Noting that MSM make up roughly four percent of all men in the country, he adds: "The hidden MSM population is significant and if we don't avert new infections the MSM epidemic could contribute significantly to the overall (HIV) prevalence rate."
Prevalence among men engaging in gay sex is 8.7 percent, nearly 10 times the norm, while incidences of other sexually-transmitted diseases are rapidly rising, indicating that fewer MSM are using condoms.
"All the messages are around heterosexual behaviour, particularly brothel-based sex work. An enormous amount of work needs to be done," said Lisle.
One of the biggest tasks is challenging Cambodia's conservative social norms that force many men to quietly seek sex with other men -- very few of whom are prostitutes -- while carrying on with lives that include marriage and family.
"There is a lot of stigma and discrimination. Culturally, economically and socially, there are all of these reasons that work against talking about sexual behaviour," said Lisle.
"In the meantime there are people seeking sex in a whole range of situations. They would never identify themselves and that makes prevention very difficult," he said.
While a barrage of public service campaigns have raised overall awareness of the causes of HIV/AIDS transmission, a shocking level of ignorance is also fueling this growing crisis among MSM, advocates say.
"MSM are at a higher risk of becoming infected with HIV because they usually don't understand how it is transmitted," says Sum Thy of the non-governmental organisation Family Health International (FHI), which has surveyed thousands of men on their sex habits.
"They think HIV only occurs among men and women who have sex and don't use condoms," he explains, but adds that the situation is slowly turning around.
At least one clinic deals almost exclusively with MSM, with the numbers seeking its services rising from a handful in 2003 to more than 200 a month last year.
Another positive sign, according to UNAIDS' Lisle, is the establishment of a national MSM working group that is trying to coordinate outreach programmes in a bid to head off new HIV infections.
The creation of the working group is a "very healthy sign that Cambodia is really starting to move to address the MSM epidemic," Lisle says. "But even if the government is providing space for this, at the same time you have to move society around.
"It's a huge journey. If we don't scale up efforts, it could be very serious."