Others have addressed the main issues you raise. Let me bring up two lesser points:
One: Watching or reading ANYTHING in class other than class-related materials is generally considered at the very least rude to the professor, even if the material itself is not offensive in any way. Teachers routinely complain about students passing notes or texting on their cell phones, even if what they're saying is "let's go out for pizza tonight".
And it seems rather foolish. You're paying thousands of dollars a year to attend this class -- or somebody is paying on your behalf. And you're not even going to pay attention?
Two: "I find it difficult to believe they do not visit any porn sites." I can't speak for your friends, but I'm am quite sure that there are many college students who never visit porn sites, period, never mind while in a school lab. Really, you need to be very careful of the thinking, "I and my friends do X, so of course everyone in the world does X." No, that doesn't follow.
I have often heard people say, "How in the world did Jones win the election? I voted for Smith. Everyone I know voted for Smith. Who voted for Jones?" The simple explanation is that you and your friends are not necessarily a representative sample of the population. Etc, I could give many other examples. Everyone in the world does not think and act just like you. Don't assume they do.
I'm 57 years old. The Internet wasn't invented yet when I was in college. But I've been working in IT for 36 years now and I can only think of one time in all those years that I ever saw someone accessing pornography at the office -- and that was someone who gave the phone number of a "phone sex" line to co-workers as a joke, telling them it was a client who had called for them and they should call back. Maybe there were some who did it discreetly and never got caught, but it is certainly not common practice in American business.