A 1953 law makes adultery a crime, but despite this, 46,000 people still signed up a week after launch.
The extramarital site has also launched in India, Hong Kong and Japan – where 120,000 users signed up after 10 days.
Speaking to AFP, Ashley Madison CEO Noel Biderman called the law “hopelessly outdated”, and said he doesn’t worry about the moral issues around launching in the country.
“I don’t struggle going to sleep at night over that.
“The point is that people are going to have affairs no matter what avenues are open or closed to them. So why not provide them with a discreet way to do it?
“It’s an alternative to affairs in the workplace, which risk exposure, humiliation and dismissal.
“If anything, our website will help keep people out of the courts.”
The law is prosecuted only on complaint, and usually results in a suspended sentence rather than jail time.
The numbers of people given jail time is also dropping – 216 in 2004, and 42 in 2008.
However, although there is a law against adultery, there is no legislation which would allow authorities to block AshleyMadison, even if they wanted to.
Song Myung-Hoon from the Korea Communications Standards Commission told APF that they are monitoring the site to see if people are using it to engage in prostitution – something which would allow them to shut the site down.
Ashley Madison saw similar opposition when they made plans to launch in Singapore, where a Facebook petition was set up calling for the government to block the site.
And in November, the Singapore Media Development Authority did enforce a ban on the site, on the grounds it promotes infidelity and disregards family values, and has rejected subsequent requests for the ban to be lifted.