The decision was called "obscene" by Karl Auerbach, former board member of Icann which approved the .xxx plan.
Mr Auerbach said Icann should be giving priority to socially positive domain name proposals rather than helping pornographers profit.
The decision was also called "dangerous" by net privacy campaigner Lauren Weinstein, who said it could provoke censorship problems for years.
Icann reluctance to approve the setting up of new net domains was creating an "artificial scarcity" said Mr Auerbach.
In light of this, Mr Auerbach said Icann should concentrate on socially constructive domains first rather than pandering to pornographers.
In his blog he asked: "Why should .xxx get precedence over schools, churches, civic groups, aboriginal communities, labour organisations, and artistic groups?"
He pointed out that more than 50 organisations have submitted proposals to Icann for new net domains but, so far, only seven of those have actually been created.
This question of how to categorise sexual websites was picked up by Lauren Weinstein, a well-known net privacy campaigner.
Also in a letter published on the IP mailing list, Mr Weinstein said "the existence of dot-ex-ex-ex is likely to create a political and litigious firestorm."
Although some outright porn sites will be happy to move to the .xxx domain, others that only have some "adult" content on their pages could be much more reluctant.
Mr Weinstein said the religious right in the US might be motivated to make any site with any "adult" content move to the .xxx domain.
Full Story .