Hmmm. I forgot – mainly because I do not like to think about it – the Svay Pak area 11 kilometers north of Phnom Penh. Also known as K11, because of the distance. That IS a big center for child prostitution despite claims of a crackdown. Ten years ago, every Western pedophile who could afford the airfare descended on Svay Pak in droves. It garnered SO much bad attention that the authorities finally had to do something, but by all accounts it’s still an active area, and I believe here you will find establishments staffed entirely by minors. 
Then there’s the main brothel area of Tuol Kauk, which is another center of child prostitution.
I’ve not been to either of the two areas above. There is absolutely NO reason whatsoever to go to Svay Pak unless you are looking specifically for an underage prostitute. In Tuol Kauk, you’re taking your life in your hands; lots of drunken ex-Khmer Rouge with chips on their shoulders picking fights at night.
It may not be a port, but I’d include Phnom Penh in a list of Seven Sinful Cities. You can reach it upriver, I guess. And it’s still an interesting place to visit even if you’re not after sleaze. The riverside area is pleasant, with the royal palace worth a look. The old Khmer Rouge torture center of Tuol Sleng (a former high school). The Killing Fields outside the city. Lots of neat old French-colonial architecture, and the streets are well laid out, thanks to the French.
