For advertisers, you are what you click on. Many websites and pretty much every catalog/sales website has tracking engines that make note of what pages you've visited and try to draw inferences from that information as to what kinds of things you might be interested in buying. People who visit sites that suggest an interest in particular types of things, particularly things that suggest you may be making changes in your spending patterns, are more likely to see ads that cater to those types of choices.
Consider cell phones. Last fall
I went and looked at cell phone providers -- particularly T-Mobile -- and researched the costs for adding my father and son to our T-Mobile account. I've been seeing cell phone ads, mostly T-Mobile ads, ever since.
I also looked at porta-pots. For two months I got toilet ads on the web pages I visited.
If your online activity doesn't suggest a specific, more valuable set of potential buying interests, you wind up getting dragnet ads. Dragnet ads are the least expensive category of ad, and that's the kind of ad a lot of these adult-themed websites like to pay for. (There's actually research that suggests that adult ads don't have a preferred demographic. Randomly-selected individuals of either gender are just as likely to click on an adult ad as more carefully targeted individuals of the same sex, so paying extra for targeted ads just doesn't make sense.)
Yes, it can be annoying, but this does give you a tool that allows you to "manage" the ads you see. In my case, I periodically visit the T-Mobile website and look at the phones they have for sale
and at their rate plans, so I see their ads a lot. I also visit HostGator, a website hosting service where some of my websites are housed, so I see their ads. Ditto some of the camping outfitters, like Cabella and Sierra Outpost. I often see Amazon ads. (Amazon . . . my world leader in toilet banner ads.)