Otherwise, it would be a a bit like a plumber fixing your boiler and catching sight of a pornographic magazine you had hidden away somewhere. Hardly sexual harassment.
I think this quite concisely summarizes the harrasment issue. In most cases it is about intent. Proving that someone intended to insult, harrass, etc. In large companies where those policies are very clearly laid out, this is often a no brainer, and all things err on the side of over-caution. BECAUSE the company is afraid of a giant lawsuit. In small businesses, these issues are often not addressed until it becomes an issue, then depending on the parties involved, can be blown off in a single conversation. Then at that point it is escalated to a legal battle. From that point on, things take on an entirely different role between employer and employee.
SO back to what to do?
My advice, don't beat around the bush. I have worked for several smaller companies and been very closely involved with the personal dealings of the owners - not by choice, but by necessity of job function.
Two ways to approach this. As some others have mentioned, since it is predominately on the weekend, mention that you have serious concerns about the safety of the network and the proliferation of viruses. Since it is your job to fix them, explain that there could be cases where you are unable to fix the problem without serious downtime, and if this spreads to the right computer or server, could suddenly become a company wide issue. Is the financial risk of downtime, loss of productivity and possible expense to replace components worth these risks?
Second possibility is to mention the possibility of what happens if others in the company think it is ok to surf porn sites. This could lead to a full time job for you to clean infected computers, explain why it is not ok for people to surf porn or other personal sites etc. Pull yourself out of the equation. Put the job squarely on his shoulders. What do you do if others are doing the same thing? Is this the sort of example he wants to be setting for his employees? Moreover, many viruses can be spread unvoulntarily - mass emails -etc. What kind of embarassment would that pose to the company to spam their mailing lists with porn emails, on a daily basis? Or worse yet, some use false headers that they scavenge from legitimate email sources - not good to invite that sort of potential embarasement on purpose.
My advice DO NOT just arbitrarily block any sites. This is inviting a confrontation. Weekend or otherwise. Bottom line is it is his company, he owns all the equipment, if he wants to infect it with viruses and pay you to clean them - that is his perogative. Don't put yourself in the position of second guessing him - just be sure to explain clearly what kind of risk his actions are posing - to himself, his company, to you, the equipment, the company's reputation, etc.
My guess if he is the sort to openly browse porn on weekends, he is a "no hassle" kinda guy, and you should be able to "straight shoot" him about his actions. If it helps, approach it from a point of doing some research on viruses, and the proliferation of them from porn sites, etc. This will at least break the ice, and probably open a channel of communication that will be more beneficial to you, him and the company.