You can exhale. You are not in trouble, and nobody is going to come and arrest you. There is no such thiing as the United Stated Criminal Law and there's no Article 161 in the US Code that relates to pornography.
In fact, if you just Google "article 161" you'll get taken to a list of scam reports about something we call the FBI virus.
Internet porn is legal unless the site portrays child pornography. But unscrupulous hackers infect porn sites with viruses that cause this message to display. In reality, this has nothing to do with the FBI but it IS a computer virus, and you need to get your computer debugged.
The FBI has been aware of this scam/virus and warns people that they have nothing to do with it here on their website:
When the FBI really wants you they show up at your house with a search warrant and they impound all of your internet access devices. They don't give you advance warning so that you can erase your hard drive or skip town. The only thing you have to worry about here is the virus, which could contain malware and allow hackers to get at your personal identifying and account information.
So take your computer to a local tech person (don't use an online company unless you have vetted them and are sure they are for real) and tell them to debug your computer. Then change all your PIN numbers and passwords. If you are computer savvy, you can post a question to our Computer Experts here as their credentials have been independently verified, and ask them to walk you through a debugging. Get that done as quickly as you can to prevent any possibility of identity theft.