Porn sites are notorious for loading computer viruses and spyware onto your computer.
At the very least I would say that a person should not be allowed to use someone else's computer to view porn, as it puts their computer at high risk for virus attacks, which can screw up your computer (and others'!) bigtime.

Needless to say, today's internet porn sites make Playboy magazine from 20 years ago look like Woman's Day magazine.