Is it normal that my dad looks at websites where you can book prostitutes?

I often catch my Dad watching porn or similar things. My parents are married for 25 years and barely have sex. I know that because would hear them since they're room is above mine. My mom also knows that he watches porn, but I think she has no problem with it and I also think it is pretty normal after being married for 25 years. Tonight my mom was working (as very often) and I went down to the kitchen because I was hungry. Our House is very open and we have a computer, which is for the whole family (mostly only my dad uses it). The point is that i saw my dad sleeping in front of it (he falls asleep very fast and also he had to work very much the last two days) and a website was opened. At first I couldn't see anything near from it but then I saw that it was a page where you could I think book prostitutes and he also had an account and people from our city where suggested (not only women but also men). I looked the website up and I saw that you also have to pay for it. I'm afraid that he has sex with these people and my mom doesn't know anything. I don't blame him for looking at that but why does he not just watch normal porn or is this normal to look at sites like these?